Add output format option

[scastlara]

Add a cli option (--output-format) that changes how the review or guide is printed in the terminal.

Possible options:

• pretty (default): current option that uses rich.
• plain: similar to the above one but without color nor rich shenanigans (useful for ci systems with no color support etc).
• json: useful for using lgtm output with another downstream tool (say, parsing the output and failing based on the score, etc.)

[scastlara]

I did not add plain because I do not think it's that important. If we want it in the future, let's create a separate issue.