Temporarily suspend PHP Warnings on invalid tags

[salcode]

Temporarily suspend PHP Warnings on invalid tags when processing Gist HTML by setting the libxml_use_internal_errors() value to true.

The original value for libxml_use_internal_errors() is restored when the processing is complete.

props @Dan0sz

See #80

Resolves #81

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document. - Note: I was unable to locate a CONTRIBUTING document for this project.