Add functions to increment/decrement context

[ethan-coe-renner]

Thanks for this great package, I use it daily!

I wanted to be able to increase/decrease the before/after context via keybindings rather than clicking through the buttons in the header of a deadgrep buffer. I didn't see a way to do this with the functions deadgrep currently exposes, so I wrote my own. I figure others might find this useful, so this PR includes that code.

(Please let me know if I've missed something and this functionality already exists).

New Functions

deadgrep--update-context does most of the work. It takes a string (which-context, which can be either "before" or "after") and an integer (value). The function updates the context denoted by which-context by adding value to the appropriate field in deadgrep--context, after ensuring that doing so won't result in a negative value.

A family of interactive functions named deadgrep-<increment/decrement>-<before/after>-context are implemented to use deadgrep--update-context to increment/decrement the before/after context by 1 line. These are intended to be bound to keys.

Keybindings

I've not added any keybindings to the deadgrep-mode-map, but in my configuration I use the following hydra:

(defhydra deadgrep-context ()
                 "context"
                 ("B" deadgrep-increment-before-context "increment before context")
                 ("b" deadgrep-decrement-before-context "decrement before context")
                 ("A" deadgrep-increment-after-context "increment after context")
                 ("a" deadgrep-decrement-after-context "decrement after context"))

I'm happy to add default keybindings if that is desired and suggestions are welcome for such keybinds.

Note: This is my first non-trivial elisp contribution to an emacs package, so please let me know if there's anything here that needs to be changed, I'm happy to do it.