XPI - Install XP Modules

This pull request implements xp-framework/rfc#270 and implements the "xpi" utility as documented:

Basic usage
--
```sh
# This will search for modules
$ xpi search query

# This will show information about a module, and its releases (newest first)
$ xpi info vendor/module

# This will install the newest release of the specified module
$ xpi add vendor/module

# This will upgrade the module to the newest release
$ xpi upgrade vendor/module

# This will list installed modules
$ xpi list
```