Add support for metadata, extensions, PATH

- Adds support for specifying description and arguments in `options.metadata`
- Adds support for searching the PATH for commands which closes #5
- Adds better support for different extensions
Formatting
Add documentation for metadata, usePath
Merge branch 'master' of github.com:mattmcmanus/node-helmsman
Add tests for metadata option
Merge pull request #11 from beaugunderson/master

Add support for metadata, extensions, PATH