mail: Fix mail-check hooks

Fixed running the hooks for `mail-check`. Command (or variable?)
substition prevented the hooks from running completely beforehand, so I
moved them to a separate function with the option remaining to call a
custom script for them through environment variables.

Ideally the env vars (`MBSYNC_PRE` and `MBSYNC_POST`) should only point
to a single script each, as more involved commands will presumably fail
to execute correctly when whitespace substition is involved.

See more explanation here:
https://unix.stackexchange.com/a/444949

mail/.local/bin/mail-check

@@ -12,8 +12,23 @@
 # https://sourceforge.net/p/isync/feature-requests/8/
 
 MBSYNC_MAX_TRIES=3
-MBSYNC_PRE="imapfilter -c /home/marty/.config/imapfilter/config.lua"
-MBSYNC_POST="notmuch new && afew --tag --new"
+
+prehook() {
+  if [ -n "$MBSYNC_PRE" ]; then
+    eval "$MBSYNC_PRE"
+    return 0
+  fi
+  imapfilter -c "${XDG_CONFIG_HOME:-/home/marty/.config}/imapfilter/config.lua"
+}
+
+posthook() {
+  if [ -n "$MBSYNC_POST" ]; then
+    eval "$MBSYNC_POST"
+    return 0
+  fi
+  notmuch new --quiet 2>/dev/null
+  afew --tag --new
+}
 
 # fail the routine and optionally send a message why
 fail() {
@@ -41,7 +56,7 @@ if [ "$1" = "raw" ]; then
 fi
 
 main() {
-  $MBSYNC_PRE
+  prehook
 
   tries=0
   while true; do
@@ -56,7 +71,7 @@ main() {
   done
   unset tries
 
-  $MBSYNC_POST
+  posthook
 }
 
 notmuch_foldertags() {