Prevent `mksession` being run within the command-line window

feat: create absolute session directory if it doesn't exist

refactor: clean up user commands

docs: document sessions.start_autosave()

feat: add sessions.start_autosave() API function

docs: remove notice

fix(#4): don't join events to a single string

docs: add request for contributions notice to readme

fix: session save on Windows when absolute = true

docs: add documentation for `absolute` config option

feat: add absolute session dir config

docs: add note on stability

docs: add link to older version for compatibility

refactor: use nvim lua api for autocommands

docs: mark last version compatible with neovim 0.6