Add customizable ignored patterns (ex. spaces-only selection)

[abougouffa]

This adds a way to exclude any arbitrary text from the highlighted selections (configured as a list of regular expressions to ignore).

The default value is set to ignore space-only selections. We can set it to nil by default so we keep the current behavior, with the user being able to customize it easily.

This PR will fixes the issue addressed by #2 when the value is set to ignore space-only regions. So, it is a more generic approach to fix this issue.