1) finish the step allowing the use of es6
- this include some tweak to web pack configuration to speed up
recompile in watch mode (in particular cache sourcemaps).
- enable eslint (error only), on obvious mistakes.
- setup babel to compile to es5 as a target.
2) Make the test pass under Casper that does not always have
`Function.prototype.bind` defined, which we cannot patch only in the
tests.
3) Write an actual shortcut editor that list and allow to modify most of
the command mode shortcut.
The logic to persist the shortcuts is a bit tricky as there are default
keyboard shortcuts, and so when you "unbind" them you need to re-unbind
them at next startup. This does not work for a few shortcut for
technical reasons: `<Esc>`, `<Shift>`, as well as `<Ctrl-Shift-P>` and `<F>`
which register asynchronously, so are not detected as "default"
shortcuts.
