Rename TokenReader to TagReader

The previous commit by this name left out a file by mistake.
Moved tags into Tags namespace
Changes to tag structure

- End tags were moved into the files of their starting tags
- Helpers.Pass() renamed to Helpers.Truth()
- RootTag deleted; it added no value beyond its BlockTag base, which is
now used instead
- Descriptions of tags improved
- BlockTag moved into the "Block tags" folder
Removed setting to ignore missing keys

A missing key in the IDictionary probably isn't normal, so the templater
no longer allows ignoring them.
Changed template file extension

Templates now use .rdt (for RainDrop Template) files instead of .rdl
(for RainDrop Language).
Updated readme

The readme hadn't been updated for a while; now it has.