Add 'parsoid' option to parserTests.txt.

Rather than overload the 'disabled' option, explicitly mark Parsoid-only
parser tests with "parsoid" in the options field.  These are disabled
by default when the PHP parser tests are run (but you could explicitly
enable them with --run-parsoid if you wished, in the same way that you
can enable other disabled tests with --run-disabled).

Document the 'php' option, which the PHP parser tests will ignore, but
will (in the future) be used to mark php-only tests which should be
ignored by the Parsoid parser.

Tweaked 'disabled' option to 'parsoid' for those tests which explicitly
call themselves parsoid-only.  I was conservative in this patch; if
the title of the test didn't explicitly mention Parsoid, I left the
test disabled rather than switch it to parsoid.

Change-Id: Id6c396f7966fcb21c1e54e222ab0c9f4e3a34dcc