[TiddlyWeb] Variable recipe support

[mahemoff]

I have private bags, dynamically created for each user who logs in.

Right now, there is no clear way to serve a recipe containing all the normal bags and recipes, along with a private bag belonging to just one user. Recursive recipe support, ie recipes containing other recipes, seems like the cleanest way to achieve it.

[cdent]

Rather than recursive recipes to satisfy the initial problem it may be better to accept expandable values in recipes, such as

/bags/$user$/tiddlers

or something similar, so changing the summary but leaving initial description for posterity.

[cdent]

This has been done in changeset:9058 as /bags/{{ user }}/tiddlers (the spaces matter)

It has some warts, as described in the commit message