Added ability to exclude datatypes from export

To skip exporting a specific of data type, exclusions can be specified in the following form:

./vendor/bin/schematic export --exclude=assetSources

Multiple exclusions can also be specified:

./vendor/bin/schematic export --exclude=assetSources --exclude=categoryGroups

Here is a list of all of the data types and their corresponding exclude parameter values:

Data Type	Exlude Parameter
Asset Sources	assetSources
Category Groups	categoryGroups
Element Indexes	elementIndexSettings
Fields	fields
Global Sets	globalSets
Locales	locales
Plugins	plugins
Plugin Data	pluginData
Sections	sections
Tag Groups	tagGroups
Users	users
User Groups	userGroups

Thanks to @spoik