Add option to specify bot (user-agent) for robots.txt

[marshvee]

We want to add an option to run the script as a specific user-agent (Googlebot, for example), so it follows the specific rules for Googlebot instead of the general ones.

We allow this by add the flag user-agent:

python main.py --parserobots --user-agent Googlebot

Resolves #87

[marshvee]

Whenever you can check this too it would be great @c4software