moved binary search to advance to a start date from datetime_filter to LogFile. It is more useful and useable from other components there.
added MaskFilter class. Takes another log file as mask and pads each event in that file. Only accepts log lines that lie within these masked areas. See #138.
extended MaskFilter to also support --mask-center as an argument. It can be 'start', 'end' or 'both'.
Added tests for mask filters.
Merge branch 'feature-138-masks' into develop. Fixes #138.