mail: Add contacts lookup script

Added `mail-searchcontacts` script, which uses notmuch to search for all
contacts of the mail database.

Can be invoked without any arguments to list *all* contacts of the
dataase in descending order of message count.
Otherwise, it will filter known contacts, first by prioritizing the
'from:' field, and otherwise by searching through the whole database for
the keywords passed in.

It will also re-format the results so that the list consists of
tab-delineated 'mail name' rows. If there is no name, the row will only
contain the bare e-mail address. This format is best accepted by aerc,
for which the script is written.

aerc uses the contacts script for its address completion in editing
e-mail headers.

Renamed `checkmail` to `mail-check` to fit the naming scheme of
`mail-[action]` for mail scripts, to better be able to group them in the
shell.

mail/.config/aerc/aerc.conf

@@ -140,7 +140,7 @@ header-layout=To|From,Subject,Cc|Bcc
 # the first field. Only the email address field is required. The second field,
 # if present, will be treated as the contact name. Additional fields are
 # ignored.
-address-book-cmd=
+address-book-cmd=mail-searchcontacts %s
 
 [filters]
 #

mail/.config/aerc/binds.conf

@@ -4,7 +4,7 @@
 gT = :prev-tab<Enter>
 gt = :next-tab<Enter>
 <C-t> = :term<Enter>
-gR = :exec checkmail<Enter>
+gR = :exec mail-check<Enter>
 
 [messages]
 q = :quit<Enter>