Grammar does not enforce whitespace after `:` token in footers

[jan-ferdinand]

As per conventional commits specification v1.0.0 §8: “Each footer MUST consist of a word token, followed by either a :<space> or <space># separator, followed by a string value.”

However, the reference grammar currently specifies:

<separator>       ::= ":" | " #"
/* missing <space>     ^^ */

This is similar to but distinct from #41. The inconsintency was pointed out in crate-ci/git-conventional#54.