Adds linux-support

[Deradon]

Without this change, I'd get the following error message when trying to install the chrome-extension:

Error message

Could not find key specification for 'command[1].suggested_key': Either specific a key for 'linux', or speify a default key.

[tapickell]

:+1: Would be nice to see this mashed in and deployed.

[tapickell]

@dutzi is this project still alive? Seems like a great tool with some support from the community.