make synopsis parser return unknown tokens
show warnings for invalid tokens when generating man pages
ignore empty synopsis tokens
refactor pattern generation
first pass at moving arg checking into SynopsisParser

Also, doesn't nest accepted parameters by type
add 'repeating' flavour
the 'repeating' flavour can also be optional
fix post generate and user generate synopses

closes #296
Merge pull request #298 from wp-cli/synopsis-lint

Synopsis linting