v1.21.4

Maintenance release

To improve the template readability the following template filters have
been renamed:

* `ext` to `file_ext`
* `stem` to `file_stem`
* `tag` to `file_sort_tag`
* `trim_tag` to `trim_file_sort_tag`
* `copy_counter` to `file_copy_counter`

The new environment variable `TPNOTE_EXTENSION_DEFAULT` allows setting the
file extension of new note files. If set, it overwrites the
`filename.extension_default` configuration file variable.