Ignore comments in CSV/TSV files by default

After importing a CSV/TSV, which has comments in the first rows marked with #, Munquet tries to use these comments as column headers. Consider providing an option or completely ignoring (by default) rows starting with "#" when trying to guess column headers. RFC4180 does not define how comments should be marked but the de-facto way is to mark comments starting with #.

An example file with comments can be generated by running wittrans (https://pypi.org/project/wittrans/) for a given-language, e.g. wittrans id -a -o tsv