Improved support for extensions

- Fixes mattmcmanus/node-helmsman#3
- Fixes mattmcmanus/node-helmsman#4
Fix typos
Merge pull request #6 from beaugunderson/extensions

Improved support for extensions
Merge pull request #7 from beaugunderson/readme

Fix typos in README.md
Adding @beaugunderson to contributors
0.2.6
Adding Badges to the readme
Create .travis.yml
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