Use a map instead of vector to store line counts.

There are a few motivations for this:
- Using a map allows for checking if line is in map. This differentiates
  unexecutable lines (such as comments) from unexecuted logical lines of
  code. "#####" is now outputted in this case, in line with gcov.
- Source files are no longer read in twice: once when storing the line
  counts, and once when outputting the data.
- Greatly simplifies the function FileInfo::addLineCount().