Fixup parsing of `{% else-%}`

[charlespwd]

This is a weird parsing ambiguity problem.

Before, our rule was the following:

identifierCharacter = alnum | "_" | "-"
liquidTagBaseCase = "{%" space* (tagName ~identifierCharacter) space* tagMarkup "-"? "%}"

Our problem was that - is both an identifier character and the character we use for stripping whitespace from the end of tags and variable output (-%} and -}}).

So {% else-%} threw an error because tagName was followed by -, which is an identifier character.

The solution is to change that rule to the following:

• Followed by an "end of tag" -}}, -%}, OR
• Not followed by an identifierCharacter

Fixes #126