Add Transient menu

As per discussion at the end of issue [#11](https://github.com/alphapapa/magit-todos/issues/11#issuecomment-401583376).

Some ideas of what a popup would be good for:
- call `magit-todos-update` (i.e. an action)
- toggle the grouping and sorting options,
- add/remove keywords,
- list of path globs to scan,
- filter found items by extra prefix or suffix patterns to found keywords (for example: "FIXME: yourname - validate input").

If the popup can save changed settings (in dir-local variables, for example), it will support settings per-repository. This can benefit users who have multiple repositories.