svtplay_dl.output: prettier progressbar generation
svtplay_dl.output: Make progress() wrap progressbar()

The progressbar() currently does not handle the case where the total size is
unknown. But with this change, progress() will internally use progressbar()
for its bar generation, while still supporting a more basic progress info when
total size is unknown.
svtplay_dl.tests.output: initial set of unit tests for progress()
Merge pull request #40 from olof/topic/output.progress_fixmes

Fix some svtplay_dl.output related fixmes