mail: Add checkmail routine

Added script to call for checking mail. Will run individual pre- and
post-hooks. (By default, imapfilter and notmuch update.)

Simple options can be changed through environment variables:

Hooks can be set to different scripts with MBSYNC_PRE and
MBSYNC_POST, respectively.

Will also run mbsync repeatedly, until it exits without error. By
default it will try running it 3 times before giving up, this can be set
with MBSYNC_MAX_TRIES.

No functional imapfilters have been added yet.

mbsync and imapfilter both adhere to the XDG base directory layout
(somewhat), checking for their option files in .config/{isync,imapfilter}
respectively.

Everything still pending changes, the mail eco-system is hard to
wrap one's head around.

mail/.config/aerc/binds.conf

@@ -4,7 +4,7 @@
 gT = :prev-tab<Enter>
 gt = :next-tab<Enter>
 <C-t> = :term<Enter>
-gR = :exec updatemail
+gR = :exec checkmail<Enter>
 
 [messages]
 q = :quit<Enter>
@@ -28,7 +28,7 @@ J = :next-folder<Enter>
 K = :prev-folder<Enter>
 <C-p> = :prev-folder<Enter>
 gi = :cf Inbox<Enter>
-gm = :cf Important<Enter>
+g' = :cf Important<Enter>
 ga = :cf Archive<Enter>
 gA = :cf All<Enter>
 gs = :cf Sent<Enter>
@@ -40,6 +40,7 @@ V = :mark -v<Enter>
 <Enter> = :view<Enter>
 l = :view<Enter>
 D = :prompt 'Really delete this message?' 'delete-message'<Enter>
+i = :modify-labels +inbox -archived -deleted<Enter>
 d = :modify-labels +deleted -inbox -archived<Enter>
 a = :modify-labels +archived -deleted -inbox<Enter>
 A = :modify-labels +archived -deleted -inbox<Enter>