Add warning for unsupported tags.

When encountering an unsupported @tag, JSDuck now prints a warning.

To achieve this, I've enhanced the doc-comment parser to only accept
@tags when they have a space in front of them.  Previously this would
have been detected as @method tag:

    /** foo@method.com */

Now it's treated as plain text as it should be. Only when separated
with spaces, the @method tag gets detected:

    /** foo @method .com */