Reworked the intruction system

Top level tags now use find: instruction to flag them for further
parsing.
append:, overwrite: and remove: are to be invoked directly in tags that
will be changed.
Tags with one of these three instructions don't get parsed further -
matching tags get either removed or overwritten with the instruction
tag's contents. append: adds the instruction tag to matching parent.
For this reason, children of these tags are to be written like normal
tags, with the exception of selectors for multiple tags.