Add Chapel parallel programming language

Chapel shows up on github in a few places (listed below). We, the chapel team, are going to move the main development repo to github later this month. There is a significant standard library implemented in chapel, which we would like to be correctly categorized.

There is a ChapelLexer in pygments. It is only available on the `default` branch, though. It is scheduled to be released with pygments version 2.0.

We picked a green that shows up in the chapel logo for the color.

Existing github repos with chapel code (that we are aware of):
* https://github.com/aroonsharma1/CHAPEL
* https://github.com/n7jti/Parallel_Computing
* https://github.com/rampion/chapel-tutorial
* https://github.com/benjcleveland/assignments
* https://github.com/chapel-lang/chapel/tree/master/modules

Chapel website: http://chapel.cray.com/

Thanks!