Enabled customized "separators" and updated documentation accordingly

[randolf]

Added a "separators" argument to the constructor to support customizable separators.

Updated the documentation to demonstrate and explain how the "separators" argument is used.

[zakame]

Thanks for the PR! Could you supply some additional tests for the newly exposed separators accessor as well? Also the current implementation is failing its tests...

[zakame]

I think you can retain seps here and just remove its init_arg option so users can set it at object construction, e.g. Hashids->new( seps => 'aeiouy' ):

has seps => (
    is  => 'rwp',
    isa => sub {
        local $_ = shift;
        croak "$_ must not have spaces" if /\s/;
    },
    default => sub {
        my @seps = qw(c f h i s t u);
        join '' => @seps, map {uc} @seps;
    },
);

[randolf]

I'm fine with using "seps" rather than the longer "separators" keyword. I also like your suggestion for this preference because, now that I think of it, it can prevent problems resulting from a somewhat common misspelling that comes out as "seperators."

[zakame]

Following the above, you won't need to split here...

[zakame]

... and instead do the split here.