This is a followup to #28

See http://yaml.org/spec/1.1/#nb-plain-char(c) and the following
productions.

This commit will allow `[http://example]`, but still fail for:
- `[:foo]`
- `[foo:]`

See https://gist.github.com/perlpunk/de2c631a7b0e9002de351a680eadb573
for all the relevant cases where this change applies.