When piping yamllint output to a file, "coloured" characters aren't
interpreted and pollute text formatting with glyphs like:
�[4m./global.yaml�[0m
�[2m1439:52�[0m �[31merror�[0m no new line character...
With this commit, stdout is checked: if it's a TTY then output is
coloured, otherwise output is simple text.
Closes: #14