Genericise implementation of key/value tags

[JeanMertz]

Right now, you can have tags that contain a colon (:) and use the string on the left of the colon as the key of the tag, and the right side be the value:

bulletin/src/main.rs

Lines 68 to 73 in e16f06e

	s.split_whitespace()
	.map(|s| s.split(':').collect())
	.filter(|v: &Vec<&str>| v.len() == 2)
	.for_each(|v| {
	values.insert(v[0].to_owned(), v[1].to_owned());
	});

Some improvements to make here:

• The character to split on should be configurable using --split-char.
• If no split char is defined, then no splitting occurs, and and empty string is used as the default key.
• Even if a split char is defined, any tag without the split char present, should be added to the map with an empty string as the default key.