Handle bracketed escapes better

Implements a POSIX class and a bracket_escape generic function to handle
escaping within character classes more robustly.  Fixes issue #11,
thanks for the bug report @briandconnelly
Split not and if_next_isnt

These functions are actually conceptually slightly different,
if_next_isnt is a test for the current match anchor, in not you want to
anchor to after the not-match.
Put capture groups around lookarounds

Also other fixes and improvements found while writing styleR.
Handle vector variables in function arguments properly
Add lookaround documentation
Document functions

- capture
- group
Documentation and tests for capture

Try to clarify documentation and add the examples to tests.
Move the capture and grouping functions to their own file
Rename POSIX to character class

It was used for more than just POSIX character classes, so the previous
name did not really make sense.
Move character_class code to it's own file, document some of the
functions.
Add exclude_range function and update character class documentation
Character class examples
Improve lookaround documentation
Tweak character class documentation
Documentation for counting functions
Add and document wildcard modifiers
Update documentation and rename a few files
Remove wildcard shortcut as I think it would not do what people expect
Document and programmically generate shortcuts

Also finish documenting the rest of the functions.
Add missing shortcut file