OptionParser: Add OptionParser, an option parser library for Vim

This is option parser library for Vim.
It can parse key-value type options (of course, value can be omitted)
Interface is inspired by OptionParser in Ruby standard library.
OptionParser: Add a spec
OptionParser: Add a document
Merge pull request #116 from rhysd/add/OptionParser

Added Vital.OptionParser