Allow storing alternates elsewhere (#90)

This change allows alternates to be stored in "$YADM_DIR/alt". The
correct path within the work tree will be symlinked.

Storing alternates within the work tree is still allowed. Both locations
will be considered when choosing an appropriate alternate file.

yadm

@@ -33,6 +33,7 @@ YADM_ENCRYPT="encrypt"
 YADM_ARCHIVE="files.gpg"
 YADM_BOOTSTRAP="bootstrap"
 YADM_HOOKS="hooks"
+YADM_ALT="alt"
 
 HOOK_COMMAND=""
 FULL_COMMAND=""
@@ -137,6 +138,11 @@ function score_file() {
   target="$1"
   filename="${target%%##*}"
   conditions="${target#*##}"
+
+  if [ "${filename#$YADM_ALT/}" != "${filename}" ]; then
+    filename="${YADM_WORK}/${filename#$YADM_ALT/}"
+  fi
+
   score=0
   IFS=',' read -ra fields <<< "$conditions"
   for field in "${fields[@]}"; do
@@ -1223,6 +1229,7 @@ function configure_paths() {
   YADM_ARCHIVE="$YADM_DIR/$YADM_ARCHIVE"
   YADM_BOOTSTRAP="$YADM_DIR/$YADM_BOOTSTRAP"
   YADM_HOOKS="$YADM_DIR/$YADM_HOOKS"
+  YADM_ALT="$YADM_DIR/$YADM_ALT"
 
   # independent overrides for paths
   if [ -n "$YADM_OVERRIDE_REPO" ]; then