`--check` should exit with code `> 0` when homoglyphs are found

[VanAxe]

Currently, mimic only converts the homoglyphs to explicit utf codepoints when running mimic --ϲheck. To allow automated testing, this behavior should change to allow programmatic detection of homoglyphs.

Having an exit code greater than 0 would allow programmatic tests.

[VanAxe]

This would also tie well into #38 .

[reinderien]

Sure; pull requests welcome.