Extract more info from a git hunk header

Added

• Added GithubDiffParser::Hunk#context which returns the context of the hunk.
  In this example, it would return "def blabla"

  @@ +1,6 -1,18 def blabla

• Added GithubDiffParser::Hunk#previous_line_count which returns the prevous line count in a hunk.
  In this example, it would return 6.

  @@ +1,6 -1,18 def blabla

• Added GithubDiffParser::Hunk#new_line_count which returns the prevous line count in a hunk.
  In this example, it would return 18.

  @@ +1,6 -1,18 def blabla

Fixed

• GithubDiffParser::Diff#previous_line_number_is_now could return a wrong value
  for the line number 1 in a file.