Let user define all actions

Having just 4 valid actions (albeit they do form a meaningful workflow)
was rather historical limitation caused by author's negligence.
User should be able to define any actions they want by simply creating
handlers for them.

Later we might want to provide a guidance (template) for that, but
that's a TODO item.

src/common.lua

     -- Handle action if it's valid
     --
     local action = os.getenv("IMAPDOMO_ACTION")
+    local file = "handlers/" .. action .. ".lua"
     local valid = {
         newmail = true,
         rewind = true,
         cleanup = true,
         migrate = true
     }
-    if not valid[action] then
-        error("invalid action: " .. action)
+    if file_exists(file) then
+        do_if_exists("init.lua")
+        dofile(file)
+    else
+        error("no handler for action: " .. action)
         return nil
     end
-    do_if_exists("init.lua")
-    do_if_exists("handlers/" .. action .. ".lua")
 end
 
 

src/imapdomo.skel

     mkusage "$@" "[options] ACTION" \
         -o                                                                    \
             "-c DIR      change to DIR before doing anything"                 \
+            "-l          list handlers and exit"                              \
             "-d          turn on debugging mode"                              \
        --                                                                     \
        "imapdomo will try to read init.lua and handlers/ACTION.lua from its"  \
        "configuration directory."                                             \
        ""                                                                     \
-       "four valid actions are understood; since you must write handler for"  \
-       "each action you want to use, the meanings below are purely a guide:"  \
-       ""                                                                     \
-       "    newmail - check for and file new mail"                            \
-       "    rewind  - re-add messages from FILTER_FAIL back to FILTER_QUEUE"  \
-       "    cleanup - delete or archive old messages"                         \
-       "    migrate - move mail between mailboxes"                            \
-       ""                                                                     \
        "See imapfilter_config(5)) for guide and API reference.  Few functions"\
        "are also available in .imapdomo/common.lua"
 }
     exit 0
 }
 
+lshandlers() {
+    #
+    # List recognized handlers
+    #
+    find "$IMAPDOMO_HOME/handlers" -name "*.lua" -printf "%f\n" \
+      | sed 's/.lua$//'
+}
+
 main() {
     local Action    # what to do
     local Debug     # true if debugging
     while true; do case $1 in
         -c) CdTo="$2"; shift 2 || usage -w "missing value to: $1" ;;
         -d) Debug=true; PRETTY_DEBUG=true; shift ;;
+        -l) lshandlers; exit ;;
         -V|--version-semver) show_semversion ;;
         --version) show_version ;;
         -*) usage -w "unknown argument: '$1'" ;;
         *)  break ;;
     esac done
     Action="$1"; shift
-    grep -qw "$Action" <<< "newmail|rewind|cleanup|migrate" \
-     || usage -w "invalid action: $Action"
+    lshandlers | grep -qw "$Action" \
+     || dle "no handler for action: $Action.lua in $IMAPDOMO_HOME/handlers"
     cmd=$(mkcmd)
     debug -v cmd
     bash -n <<<"$cmd" || die