Allow specifying backslash-escaped delimiters

[aotimme]

This change makes two changes to how delimiters are specified:

1. It allows other backslash-escaped delimiters, such as those that represent hexadecimal or unicode characters (like \x01 per How to change to delim from "\x01" to others? #54). Previously we had a special case to handle the tab-delimiter \t.
2. It will now exit with an error if the provided delimiter does not evaluate to exactly 1 rune. Previously it would silently accept it, which is not great since it could lead to some strange behavior for users.

Fixes #54