Added system for encoding functions applied when converting to HTML

At the moment only obfuscation is done with such function, but other
functions can be added in the future.
Simplified creation of HTML tag for a tiddler

Use join and interpolation instead of appending to a string. This is
faster and (in my opinion) clearer as well.

'self.pos' is always defined, no need to test for it.
'now' wasn't used at all.
Made position parsing more robust

Check that there are 2 coordinates.
If one of them is not an integer, ignore them instead of breaking.