v0.9.1

MetTranslate WP 0.9.1

Housekeeping and packaging release. Moves the settings class into
includes/core/, where the rest of the plugin's shared code lives.
Fixes a release zip bug where the DOCS/ folder would have shipped
inside the plugin after a folder rename made the workflow's old
exclude pattern stop matching. Adds WordPress Coding Standards
tooling. No functional changes.