theme set command should give error when index isn't provided

[pastly]

./bm theme set terminal should provide an error message about how you should be using an index instead of a theme name. The correct command is ./bm theme set 2 (or similar).

[pastly]

Adding the following isn't good enough because while it handles ./bm theme set 9999 correctly, it doesn't handle ./bm theme set terminal correctly.

[[ "${THEMES[$2]}" == "" ]] &&
	echo "'$2' is not a valid theme index. Try ./bm list" &&
	exit 1

[pastly]

A regex to check if the index is a valid int would do it, I think. https://stackoverflow.com/questions/806906/how-do-i-test-if-a-variable-is-a-number-in-bash