This adds the new validation columns as requested by @raylim and adds extra
tests to confirm they are correct. The extra validation columns are useful for
seeing exactly which mutation has been validated. Without it, it is unclear
whether an indel or a snv was validated.

Main script adds extra validation columns. Code refactoring has been done
mainly for readability and more easy testing. The mpileup_parser now only
removes a single character from the quality string. Three new tests have been
added to confirm that both mpileup parser and the construction of the
subsequent extended dataframe are working correctly.