Highlight explicit keyword arguments

[agentx3]

Basically same as numirias#78 (comment)

I think it would be nice to have the left-hand side keywords highlighted. Many thanks.

[wookayin]

I've added a new highlight group semshiKeywordArgument. Please try it out!

[agentx3]

Awesome! Thanks for implementing it

[wookayin]

I will close this by #10. NOTE: I might change the highlight group to semshiKeyword (still undecided).

[wookayin]

It was decided to be semshiKeywordArgument.