Runtime control of `mime_type_is_text()` overrides

[groner]

The issue of being unable to paste into a terminal keeps coming up with various application/whatever mime-types.

See #44, #46, #73, #102, #103
Also related #97, #143

Probably #97 would solve this once and for all, but that seems to be stalled.

The existing system of hard-coded text equivalent mime-types could be incrementally improved to allow populated the list of text equivalent mime-types at runtime. I imagine wl-copy would look for a user file and a system file.

[rocketraman]

Another one: #233