Cobertura conciseness and summary option

[rshearman]

• Add --print-summary option Add a --print-summary option along the lines
  of what gcovr supports and outputting in a compatible format. The number
  of functions covered isn't output (it is with gcovr) as this is more
  complicated to calculate and it's unclear if it would be useful.

  Fixes output coverage summary (gitlab integration) #556

• Make cobertura output more concise and add cobertura-pretty output option
  Some parsers of cobertura coverage files places limits on the file size
  (e.g. GitLab - 10MB), so being as concise as possible is advantageous.
  For this reason, change the cobertura output to not add newlines and
  indent between XML elements, and add a new output type cobertura-pretty
  for cases where the newlines and indent is desired.

  Changing the format for the existing cobertura output type rather than
  keeping it the same and introducing a new concise output type was opted
  for since cobertura is designed to be machine-readable and most of the
  time users would not need to inspect the file manually.

  This results in a 42% reduction in output file size for a repository of
  mine.

• Make output of plain lines in cobertura reports more concise For plain
  lines (without branch information) then there's no need to write out
  opening and closing tags for the line element. It is more concise to
  output an empty element, and conciseness is important to avoid hitting
  processing limits from certain services that parse the output earlier
  than is necessary.

[marco-c]

What if we have "summary" as one of the possible output types instead of adding a --print-summary option?

[marco-c]

Thanks @rshearman, I've merged the two commits related to Cobertura.