Support command files having extensions

[beaugunderson]

I have subcommands named <prefix>-<command>.js and helmsman includes the .js extension in the name of the command. It would be nice if it stripped the extension.

[beaugunderson]

Two reasons this would be nice:

1. During development if you've symlinked your node module using npm link you'll get commands with extensions even if you've properly specified them in your package.json's bin hash without them.
2. People may want to add their own commands (in their PATH, which is another issue I'll file separately) that have extensions

[mattmcmanus]

Good call. I simply haven't come across that yet.

When generating the list of commands it would be pretty trivial to wrap the commands in a path.basename(command, path.extname(command))