Handle non-printable Vimium key mappings in insert mode

If I focus a textbox, it obviously makes sense for Vimium not to interpret A or whatever as a command. However, if I've mapped ctrl-g to a command, it seems reasonable for Vimium to continue to capture that keystroke, even within a textbox, rather than having Chrome's search bar come up.

Introducing an imap command seems like the most comprehensive solution, since that would allow a small number of mappings to be configured to work in text while the rest remain disabled. A "nimap"(normal-and-insert-map) would also be useful as it would avoid having to specify these commands twice.

This is prompted largely by the existence of auto-focusing textareas. Facebook Messenger in particular refocuses the textarea even after clicking off it into the chat.

Start by tracing Vimium's existing insert-mode key handling and key-mapping commands to understand where text-input suppression is applied. Done means supporting configured non-printable mappings in text fields, with an imap command and optionally a combined nimap command, while leaving ordinary mappings disabled there.