Fix download saver on Firefox

Turns out that Firefox needs the link to be in the document before it
will honour the click on it.
Make a separate batch file for running tests

Makes it easier to run the tests without going through the build steps
first
Fix problem with non-shadow stylesheets not being recognised at startup
Fix typo
Add a batch file for splitting a TW file into tiddlers
Refactor the filter mechanism

Long overdue rewrite to make it simpler, and break the filter operators
out into individual modules.