Add `taf dep` option

[jayadamsmorgan]

taf dep would be used for downloading different dependencies such as w3c webdrivers, and other tools needed for libs/testing in general.

[jayadamsmorgan]

some of the webdrivers are not supposed to be available for installation, such as "safaridriver" because it should be just present on the system or "iedriver" if you are not on windows, etc

Mostly this is needed for "chromedriver", "geckodriver" and/or other dependencies that might appear in the future.

Thinking of an example pipeline for webdrivers, I think we should first check if the driver is present in cache (~/.taf/dependencies/something...) if not then we try to execute the driver installed system-wide, if it's also not there we should give an error indicating that webdriver has to be installed either with taf dep install something or manually system-wide.

[jayadamsmorgan]

This is good to have but not really necessary, so this one is probably going to be added after 1.0.0 release