Document the `--no-color` option in Readme

[cphyc]

Hi,
For the sake of parsing the output of textidote more easily, it would be useful to be able to remove colors from the output when using the singleline output mode.

[sylvainhalle]

Hi! You can already do this by using the --no-color command line switch. This argument is not mentioned in the Readme though!

[cphyc]

Great! That's indeed the solution to my issue. Should the issue be closed?