Add options to pretty print the output XML

[c4software]

Add xmllint to produce an human readable XML.

Ref to #26

[c4software]

With the current implementation the most simple is too run xmlint manually via pipe

[c4software]

Example :

python3 main.py --domain https://blog.lesite.us --images --parserobots | xmllint --format -

[c4software]

Added in the README