Add new `check_source` check to check source code as a string

[jiegillet]

Closes part 2 of #169. This one came together faster than expected. It might be easiest to review commit by commit.

If it gets approved, I will also add it the docs and recipe (with warnings to only use this if other techniques can't work).

[angelikatyborska]

Let's merge! I'm waiting on the docs and recipes update in a new PR :)