Add APPEND_MODE to import-osm (#365)

If **APPEND_MODE** is set to true, `import-osm` can used to import multiple areas.

The flag `-overwritecache` always clean up the cache and remove the existing data. With `-appendcache` the data will appended.

#### Usage:
To use it with `openmaptiles` it is enough to add the variable in the `.env`:
```
APPEND_MODE=true
```

Co-authored-by: Yuri Astrakhan <[email protected]>

bin/import-osm

@@ -14,29 +14,42 @@ export PGPORT="${POSTGRES_PORT:-${PGPORT:-5432}}"
 
 
 : "${DIFF_MODE:=true}"
+: "${APPEND_MODE:=false}"
 
 
 function import_pbf() {
     local pbf_file="$1"
-    local diff_flag=""
+
+    local extra_flags=""
+    local action_msg=""
+    local mode_msg=""
+
     if [ "$DIFF_MODE" = true ]; then
-        diff_flag="-diff"
-        echo "Importing $pbf_file into $PGHOST:$PGPORT/$PGDATABASE using Imposm in DIFF mode..."
+        extra_flags="$extra_flags -diff"
+        mode_msg="NORMAL"
+    else
+        mode_msg="DIFF"
+    fi
+
+    if [ "$APPEND_MODE" = true ]; then
+        extra_flags="$extra_flags -appendcache"
+        action_msg="Appending"
     else
-        echo "Importing $pbf_file into $PGHOST:$PGPORT/$PGDATABASE using Imposm in NORMAL mode..."
+        extra_flags="$extra_flags -overwritecache"
+        action_msg="Importing"
     fi
 
+    echo "$action_msg $pbf_file into $PGHOST:$PGPORT/$PGDATABASE using Imposm in $mode_msg mode..."
     imposm import \
         -connection "postgis://$PGUSER:$PGPASSWORD@$PGHOST:$PGPORT/$PGDATABASE" \
         -mapping "${IMPOSM_MAPPING_FILE:?}" \
-        -overwritecache \
         -diffdir "${IMPOSM_DIFF_DIR:?}" \
         -cachedir "${IMPOSM_CACHE_DIR:?}" \
         -read "$pbf_file" \
         -deployproduction \
         -write \
-        $diff_flag \
-        -config "${IMPOSM_CONFIG_FILE:?}"
+        -config "${IMPOSM_CONFIG_FILE:?}" \
+        $extra_flags
 }
 
 function import_osm_with_first_pbf() {