WIP: add basic implementation of global filters

[porterjamesj]

First pass at the feature suggested in #260. Not sure if this is something you want or if there's a better way to do it, just opening up a discussion.

[Fuco1]

OKey, let's make this major-mode aware too. A format I quite like is like so:

(foo bar (emacs-lisp-mode baz bar) quux (c-mode flux tux) ...)

To get major mode specific data, you do (assoc major-mode data), to get global data you do (-remove 'listp data)

Can you implement something like that?

You can see the customize type declaration for similar setting here: https://github.com/Fuco1/guide-key/commit/627276e87f3816eac8fed8ff9514adbe7536541b#diff-cedbd6db45d2fc2bbf99bcdb1eabb601R179

[porterjamesj]

Sorry for delay; I was in the middle of finals week and procrastinating with Emacs stuff and then had to actually go do school things. I'm happy to implement whatever design you think is good; but I'm a bit confused by (foo bar (emacs-lisp-mode baz bar) quux (c-mode flux tux) ...). What would this look like plugging in names appropriate for this feature instead of the placeholders?

[Fuco1]

Well that was just a suggestion, I don't particularly care about the format, but it should be possible to specify these globally and per major mode, and there should be some nice customization interface.

Since I'm lazy to produce the list you've asked for, here's the same datastructure used in guide-key package---instead of keybindings imagine the filter functions:

`("C-x r" "C-x 4" "C-x j" "C-x p" (calc-mode "V") (dired-mode "/" "*" "C-t") (ibuffer-mode "/" "*" "%"))

[porterjamesj]

Ah I see what you mean now. Yeah I can do that.