fix(config): update default keymaps to match vimdoc (#116)

Problem: four keymap defaults in `config.lua` still used the old
deprecated keys (`!`, `D`, `U`, `F`) while `doc/pending.txt` documents
the `g`-prefixed replacements (`g!`, `gd`, `gz`, `gf`).

Solution: update `priority`, `date`, `undo`, and `filter` defaults to
`g!`, `gd`, `gz`, and `gf` respectively.

lua/pending/config.lua

@@ -117,10 +117,10 @@ local defaults = {
     close = 'q',
     toggle = '<CR>',
     view = '<Tab>',
-    priority = '!',
-    date = 'D',
-    undo = 'U',
-    filter = 'F',
+    priority = 'g!',
+    date = 'gd',
+    undo = 'gz',
+    filter = 'gf',
     open_line = 'o',
     open_line_above = 'O',
     a_task = 'at',