Implement support for command argument separators.

[NCPlayz]

Summary

This is an adaptation of the now defunct PR - #645. It allows for two ways of command customization:

• Separator: Essentially lets you separate the arguments using something other than , requiring the use of quotes for multiple word arguments. ?foo a b, c, d, e -> ('a b', 'c', 'd', 'e')
• Encapsulator: Another way of using quotes. ?bar a b [c d e] f g -> ('a', 'b', 'c d e', 'f', 'g')

As this changes quite a bit of the parsing of commands behind the scenes, I will try to test this as extensively as possible before marking it ready for review.

Improvement list

• Fix parentheses in docstrings
• Use c.isspace instead of hardcoded space check
• Check if there is an encapsulator -> don't use a hardcoded dict to replace it
• Validate the objects given as parameters for Separator and Encapsulator.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[Rapptz]

Looks mostly fine aside from the questions I poised down there.

I'm going to slate this for 1.4 release so if you could add those versionadded tags I'd be grateful.