Add lexer for Solidity

[Anteru]

(Original issue 1510 created by zzzannen on 2019-04-27T15:47:23.845916+00:00)

See also PR 812.

[Anteru]

(Original comment by zzzannen on 2019-04-27T17:23:54.690740+00:00)

Hi Georg, I've created this issue in case it's needed in order to track the process of adding a Solidity lexer to pygments.

PR 812 implements the lexer. Please let me know (or point to a Contributing Readme) if there's anything missing/incorrect from the code I've added.