Support all mappable keys in passkeys and treat exit insert mode as a command

This PR combines #1140 and #1368, and adds a tweak to support passkeys for `exitInsertMode`. It does the majority of what is outlined in [this comment](https://github.com/philc/vimium/pull/1211#issuecomment-67776693) with a view to making #1211 mergable and enabled by default.

Specifically, this PR:
* adds passkey support for all mappable keys, with multi-character key representations recognised when delimited with spaces (via #1368).
* adds `exitInsertMode` and `exitInsertModeAndPassKey` commands (via #1140), which fixes #301 and starts work on resolving #1176.
* enable passkeys for the `exitInsertMode`/`exitInsertModeAndPassKey` commands.