Bugfix for: Links with "_" in the domain name are not regarded as links

[ZibanPirate]

This PR fixed the bug reported in #95

The change is adding a _ in src_pseudo_letter regEx, to be able to detect links such as:

• api_stage.dzcode.io
• api_stage.dz_code.io

which are valid links, see: https://stackoverflow.com/a/2183140/8113942

type of change:

• [x] non-breaking changes