Added check if contains option.

[Wineh]

Check if a certain term is present in line and remove the line if this is the case. This option is needed for chases which cannot be solved with the regex, starts-with or ends-with.

[zyronix]

Those cases can be solved using regex:

remove_this_one
thisoneaswell
butnotthisone

python3 bin/demeuk.py -i test.txt --check-regex '^[^_]+$','^[^this]'
butnotthisone

I'll add this example to the regex docs :). Still think this is a good pull request, as the syntax of your options is a lot easier.