multi-format convert (#362): simpler validation

This removes the explicit config validation check and collapses it into the
config read step (which is the intended convenience of Confit's API).
#380: correctly skip no-op transcodes

The format key is now the (lower-cased) format name string used by beets,
which means we can precisely detect which transcodes would be unnecessary. To
facilitate this, I added an ALIASES dict which allows more convenient names to
work for this (e.g., "wma" is easier to remember than "windows media").
convert: log transcoding command
#380: shortcut/compatibility options

These config options make it easier to customize the command (no need to make
a single-element formats dict). And the opt config option provides backwards
compatibility with the previous style.
convert: fix outdated reference, col wrap
#380: Template.safe_substitute

This way, you almost never need to use the $$ escape sequence.