Finished `main` function

Let's actually do something with that big YAML file we've been putting
together.
Fixed directory creation

The script now runs successfully, which is nice.
Fixed incorrect start time entry (and handling)

Out of habit, since time stamps are displayed this way in most
players, I added a leading zero to one of the segments' start time's
seconds value. This caused it to get parsed as a string instead of as
an integer.

I fixed the incorrect entry, but I also ensured the script would try
to parse these values as numbers (floats for safety's sake (and in
case someone wants to try adding fractional seconds in the future)).
Added flags for overwriting existing MP3s (or not)