Tags Plugin, first version

Several things have become clear writing this plugin.

1. Some sort of method to standardize tags will be helpful. Did I use "plugin" or "plugins" last time?
2. Getting tags working and getting tags complete are different tasks. I have tags working now, complete will include non-ASCII tags working (currently they display, but won't search) and more advanced searching like TAG or TAG, TAG and TAG, TAG without TAG, and no tags at all. Even if the automatically generated links won't include that, I'll want those searches for my own use.
3. The limits of the default interpolation become more obvious. I can set a a string to be included, but I can't have a template that includes a loop over an array, to e.g. have the top twenty tags listed in desktop view but only five in mobile view.

I don't plan to fix interpolation any time soon, but the other two I can see in my near future. I will probably code up a post composer of some sort to help standardize tags. With that I can foresee also making the post composer do preliminary HTML formatting from a simple markup language, and hooking the composer up to a cellphone friendly CGI page.

Fixing the tags plugin to move beyond "MVP" — minimum viable product — particularly for non-ASCII tags will also be a relatively high priority. Right now tags can be (roughly) /^[a-zA-Z0-9_-]+$/, with the caveat that I block leading hyphens (will probably want that available for tag negation searches) and I allow whitespace in tags, but map that to _ (underscore) for searches, so there will be no way to distinguish between this_tag and this tag.

But here it is: aaa_tags.

First off tags for any post are saved in a file with the same name but a different (configurable) extension. One tag per line. Lines starting with # are comments. Leading and trailing whitespace removed, and internal whitespace is normalized to a single space.

It creates a interpolation variable for a configurable number of top tags, I'm going to be using that instead of the old categories. (And I used the categories and subcategories to seed the tags on all the old posts.) It also creates a tags variable for interpolation just before each story is processed. Both of those make the tags shown into search links.

I've named it starting with "aaa" because I want it to have access to the %files and %others lists before any filter() edits them. This way I can build a complete list of unfiltered tags. This also means that tag search filters happen with a very high priority, which isn't as necessary. Just running that any time before pagination kicks in would have been fine.

With this plugin, I'm retiring my use of the categories, prettycategory, menu, and breadcrumbs plugins. That's leaving me pretty close to 100% plugins I wrote (or *cough*paginateqz*cough* rewrote).