Allow white space between '=' and delimiter.

Currently, qt-mustache will not allow any white space between the =
signs and the start/end delimiter in set delimiter tags. It will
treat such whitespace as being part of the delimiters and yield the
error "Custom delimiters may not contain '=' or spaces.".

Although the specification reads

    "The tag's content MUST be any two non-whitespace sequences
     (separated by whitespace) EXCEPT an equals sign ('=') followed
     by the current closing delimiter." [1],

apparently such white space should not be stripped, and e.g

    {{= [ ] =}}

should be allowed [2]. The spec is slightly ambiguos here, but
that is apprently the intention.

This patch changes `readSetDelimiter(..)` to skip across such
white space. As a result the "Pair with Padding" test case in
delimiters.json now passes, since it has such a set delimiter tag.
The "Sections" test case in the same file have one as well, and
would also pass now if we only handled standalone tags properly.

[1] https://github.com/mustache/spec/blob/v1.1.2/specs/delimiters.yml#L5-L7
[2] https://github.com/mustache/spec/issues/29