munge_filename isn't idempotent for filenames containing spaces

Applying `lib.munge.munge_filename` multiple times to its own output doesn't always produce the same result for filenames that originally have spaces.

In the first pass spaces are replaced with hyphens. In subsequent passes, hyphens are removed.
Could you add this real example? It shows how it deals with a common thing in titles: ' - '
```
>>> munge.munge_title_to_name('10cm - 50cm Near InfraRed (NI) Digital Aerial Photography (AfA142)')
'10cm--50cm-near-infrared-ni-digital-aerial-photography-afa142'
```