config: Warn on unknown keys and add completions

[jerry-skydio]

This print a message if there is an unknown section or key in the config
file, to help catch errors.

Also adds tab complete for all revup config keys.

[jerry-skydio]

Reviews in this chain:
└#234 config: Warn on unknown keys and add completions

[jerry-skydio]

#	head	base	diff	date	summary
0	d8442e39	e7e87173	diff	Apr 23 15:54 PM	4 files changed, 64 insertions(+), 14 deletions(-)
1	cde8727e	e7e87173	diff	Apr 23 16:31 PM	3 files changed, 16 insertions(+), 15 deletions(-)