Warn when invalid guide name is used.

Related to: #374
Redesign StandardTagParser.

Add :default and :optional config options and only return these
fields when these configs present.  With this we now also allow
the syntax for default values without the optionality-braces:

    @mytag {Type} name=<some funky default value>

Added a separate testsuite and updated all calls to
Scanner#standard_tag().
Merge branch 'master' into doc-tags