Allow a config file to define custom hook scripts

[ahw]

The user likely already has some build framework in place and command line tools for running. VimHooks should allow the option of a per-project config file that maps hook naming patterns to custom commands or scripts.

{
    "js.bufwritepost.vimhook": "make build",
    "js.vimleave.vimhook": "make deploy"
}

Or perhaps this would be more clear:

[{
    "pattern": "*.js",
    "event": "bufwritepost",
    "cmd": "make build"
}, {
    "pattern": "*.es6.js",
    "event": "bufwritepost",
    "cmd": "make compile"
}]