Issue #3372

Reference: https://groups.google.com/forum/#!msg/codemirror/uRUuqXimqj0/mn3Lmdkab18J
Rewritten to match the style of the Verilog mode

Closes #3365

Closes #3336

Closes #3345

Issue #3346

Closes #3340

Closes #3339

And add a styleDefs option to turn it off.

Issue #3332

Issue #3332

Issue #3325

Issue #3321

Closes #3307

Original code by Evan Czaplicki

Closes #3295

Since May 5th. `async` and `await` are Python3 keywords.
cf https://www.python.org/dev/peps/pep-0492/

The new version of Python is still in beta, but start to get some use.
And getting the correct highlighting out will help people not use
`async` and `await` as variable name.

Closes #3276

And a few minimal tests for the mode

Enable it for the languages that have switch.

Issue #3252

Issue #3253