Templates

This implements a minimalist template format called *[TempleTcl](http://wiki.tcl.tk/18175)*.  The implementation is in demo/template.t.

I added some commands:

* incr
* append
* error

However, in order to make TempleTcl work, I had to modify the `string range` command to more closely match tcl's behavior by including the character indexed by `last`.  I'm not sure if we want `string range` to follow the slicing behavior in go, or if we want it to follow the tcl behavior, though.