Detect image protocol instead of relying on viuer

[mfontanini]

This PR:

• Detects the terminal emulator we're using.
• Figures out which image protocol to use.
• Turns the --image-protocol cli arg from a hint into a respected value, at least for the kitty and iterm protocols.

The main motivation for this is that viuer has limited detection, e.g. it doesn't know wezterm supports the kitty protocol so now, while it still uses the iterm one by default, you can pass in --image-protocol kitty-local when using wezterm and it will be respected. This is also some ground work to be able to work on #72 next.