Zeal support added

Ref #31 

This is a little bit of a hackish way of going about things, but I basically just check to see if the current OS is Linux, and use the Zeal specific way of launching commands (it doesn't support Dash's URI handler, unfortunately). 

Another caveat - Zeal only supports searching a single docset (jkozera/zeal#77), so having multiple keywords may be impossible atm. Perhaps in the future. So right now, we're just picking the first keyword.