Refactor parser with PikaParser.jl

[caleb-allen]

Parsing of vim commands is pretty fragile and difficult to work with in the package, as it's essentially just matching input against a list of really gnarly regular expressions. The more complex the commands, the more unruly the current solution gets. For example, in order to implement registers, many (but not all) commands must be able to be prepended with "x.

Re-implementing the parsing stage with a more robust solution would make it easier to add or change more complex features. PikaParser.jl takes a really interesting approach. At best, this can replace parse.jl, and at worst it will be an interesting experiment and learning opportunity.