Allowing underscore and hyphen in any position for gravity parseList

[rdwebdesign]

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What does this PR aim to accomplish?:

This PR allows parsing domains using hyphens and/or underscores in non-standard positions, allowing Pi-hole to block them.

Background:
The RFC standards allow underscores and hyphens only in specific positions on a domain name.
The gravity_parseList() function discards domains not following the rules defined these standards, but there are real domains using these patterns.
Some lists contain these non-standard domains, but Pi-hole is currently discarding/ignoring them.

How does this PR accomplish the above?:

Using a less restrictive pattern.

Link documentation PRs if any are needed to support this PR:

none

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