Add support for check-sieve

**Name:** check-sieve
**URL:** https://github.com/dburkart/check-sieve

`check-sieve` is a syntax checker for the [Sieve mail filtering language](https://en.wikipedia.org/wiki/Sieve_(mail_filtering_language)).
It is rather limited as it only reports syntax errors without any hints on how to solve them, but it is useful nonetheless, because boy does it suck to change your filters only to find out you missed a semicolon and now all filters are broken.