This commit adds support for job control (running processes
asynchronously from plugins) with the JobStart, JobSend, and JobStop
functions (copying neovim's job control).
This commit also makes the linter plugin work asynchronously, so the
editor won't be frozen while the linter checks your code for errors.
Plugins can now create their own commands using the `MakeCommand`
function. Plugins can also now create their own keybindings with the
`BindKey` function. See the go plugin for an example of `MakeCommand`.
