Add a validating syntax parser.

Closes square#46. Fixes square#47.

Prior to this commit the gem failed to validate the RRULE's syntax:

* If a keyword was misspelled, it would *silently* be ignored
* If a keyword appeared multiple times only the last occurance was used
* If the mutually exclusive keywords COUNT and UNTIL were used together
  then no exception was raised

```ruby
> rrule = RRule.parse('FREQ=DAILY;COUNT=3;COUNT=7;UNTIL=20200302;INTERVALLLLL=9')
 => #<RRule::Rule:0x000055db33885018 ...
> rrule.instance_variable_get(:@options)
 => {:interval=>1, :wkst=>1, :freq=>"DAILY", :count=>7, :until=>2020-03-02 00:00:00 +0100, ...
```

Therefore we add a validating parser with comprehensive error messages:

```ruby
> rrule = RRule.parse('FREQ=DAILY;COUNT=3;COUNT=7;UNTIL=20200302;INTERVALLLLL=9')
Traceback (most recent call last):
...
RRule::InvalidRRule (SyntaxError)

* unknown keyword 'INTERVALLLLL' at position 42:

        'FREQ=DAILY;COUNT=3;COUNT=7;UNTIL=20200302;INTERVALLLLL=9'
                                                   ^^^^^^^^^^^^
* keyword 'COUNT' appeared more than once at positions 11 and 19:

        'FREQ=DAILY;COUNT=3;COUNT=7;UNTIL=20200302;INTERVALLLLL=9'
                    ^^^^^   ^^^^^
* keywords 'COUNT' and 'UNTIL' are mutually exclusive at positions 11, 19 and 27:

        'FREQ=DAILY;COUNT=3;COUNT=7;UNTIL=20200302;INTERVALLLLL=9'
                    ^^^^^   ^^^^^   ^^^^^
```

README.md

@@ -2,7 +2,7 @@
 
 ## Description
 
-rrule is a minimalist library for expanding RRULEs, with a goal of being fully compliant with [iCalendar spec](https://tools.ietf.org/html/rfc2445).
+rrule is a minimalist library for expanding RRULEs, with a goal of being fully compliant with [iCalendar spec](https://tools.ietf.org/html/rfc5545).
 
 ## Examples
 
@@ -62,6 +62,34 @@ rrule.all
 => [2016-07-01 00:00:00 -0700, 2016-07-03 00:00:00 -0700]
 ```
 
+### ActiveModel validator
+
+In every project using `ActiveModel` the `RruleValidator` will automatically be loaded and can be used like this
+
+```ruby
+class MyEvent
+  include ActiveModel::Validations
+
+  attr_reader :recurrence_rule
+
+  def initialize(recurrence_rule)
+    @recurrence_rule = recurrence_rule
+  end
+
+  validates :recurrence_rule, rrule: true
+end
+```
+
+or in a Rails app
+
+```ruby
+class MyEvent < ApplicationRecord
+  field :recurrence_rule, type: String
+
+  validates :recurrence_rule, rrule: true
+end
+```
+
 ## License
 
 Copyright 2018 Square Inc.

lib/rrule.rb

@@ -3,6 +3,7 @@
 require 'active_support/all'
 
 module RRule
+  autoload :Parser, 'rrule/parser'
   autoload :Rule, 'rrule/rule'
   autoload :Context, 'rrule/context'
   autoload :Weekday, 'rrule/weekday'
@@ -24,11 +25,48 @@ module RRule
   autoload :AllOccurrences, 'rrule/generators/all_occurrences'
   autoload :BySetPosition, 'rrule/generators/by_set_position'
 
+  KEYWORDS = %w[
+    FREQ
+    UNTIL
+    COUNT
+    INTERVAL
+    BYSECOND
+    BYMINUTE
+    BYHOUR
+    BYDAY
+    BYMONTHDAY
+    BYYEARDAY
+    BYWEEKNO
+    BYMONTH
+    BYSETPOS
+    WKST
+  ].freeze
+
+  FREQUENCIES = %w[
+    SECONDLY
+    MINUTELY
+    HOURLY
+    DAILY
+    WEEKLY
+    MONTHLY
+    YEARLY
+  ].freeze
+
   WEEKDAYS = %w[SU MO TU WE TH FR SA].freeze
 
   def self.parse(rrule, **options)
     Rule.new(rrule, **options)
   end
 
+  def self.valid?(rrule)
+    Parser.parse!(rrule)
+
+    true
+  rescue InvalidRRule
+    false
+  end
+
   class InvalidRRule < StandardError; end
 end
+
+autoload(:RruleValidator, 'rrule_validator')

lib/rrule/frequencies/frequency.rb

@@ -47,7 +47,7 @@ def self.for_options(options)
       when 'YEARLY'
         Yearly
       else
-        raise InvalidRRule, 'Valid FREQ value is required'
+        raise NotImplementedError, "Frequency '#{options[:freq]}' not implemented"
       end
     end