Add option that allows checking installation

[plroebuck]

Existing setupCmd somewhat determines what command/binary would be used on specific platforms.

Later, it throws an error for unsupported platforms, if no cmd is assigned.

Augment this with some method that would allow determining if that binary is actually installed on supported platform to allow runtime checking prior to usage. If not, throw error with "growl support available but not installed" and reference the appropriate platform-specific portion of your installation instructions (e.g., https://github.com/tj/node-growl/blob/master/Readme.md#mac-os-x-darwin for macOS).

[plroebuck]

ping @deiga @jbnicolai

[deiga]

@plroebuck That sounds reasonable, feel free to make a PR