Environment variables

Introduced in toot v0.40.0

Toot allows setting defaults for parameters via environment variables.

Environment variables should be named TOOT_<COMMAND_NAME>_<OPTION_NAME>.

Command with option	Environment variable
`toot --color`	`TOOT_COLOR=true`
`toot --no-color`	`TOOT_COLOR=false`
`toot post --editor vim`	`TOOT_POST_EDITOR=vim`
`toot post --visibility unlisted`	`TOOT_POST_VISIBILITY=unlisted`
`toot tui --media-viewer feh`	`TOOT_TUI_MEDIA_VIEWER=feh`

Note that these can also be set via the settings file.