Template syntax
---------------

There are two modes: text mode and function mode. The brace characters "{" 
and "}" are the only reserved characters. Either one of them will switch from
text mode to function mode wherever they appear, no exceptions. 

In text mode, all characters are passed through to the output. In function
mode, text is split into tokens, delimited either by whitespace or by 
matching pairs of braces. The first token is taken to be a function name. The
other tokens are first processed in function mode themselves, then they are 
passed to the named function as parameters. The return value of the function
is passed through to the output.

Example:
   {escape {"hello"}}

First brace switches to function mode. The function name is escape, the first
and only parameter is {"hello"}. This parameter is executed. The braces around
the parameter cause the parser to switch to text mode, thus the string "hello",
including the quotes, is passed back and used as an argument to the escape 
function. 

Example:
   {if title {<h1>{title}</h1>}}

The function name is "if". The first parameter is the result of calling the 
function "title". The second parameter is a level 1 HTML heading containing
the result of the function "title". "if" is a built-in function which will 
return the second parameter only if the first is non-blank, so the effect of
this is to return a heading element only if a title exists.

As a shortcut for generation of HTML attributes, if a function mode segment is
surrounded by double quotes, quote characters in the return value will be 
escaped. This only applies if the quote character immediately precedes the 
opening brace, and immediately follows the closing brace, with no whitespace.

User callback functions are defined by passing a function object to the 
template processor. Function names appearing in the text are first checked
against built-in function names, then against the method names in the function
object. The function object forms a sandbox for execution of the template, so 
security-conscious users may wish to avoid including functions that allow
arbitrary filesystem access or code execution.

The callback function will receive its parameters as strings. If the 
result of the function depends only on the arguments, and certain things 
understood to be "static", such as the source code, then the callback function
should return a string. If the result depends on other things, then the function
should call cbt_value() to get a return value:

   return cbt_value( $text, $deps );

where $deps is an array of string tokens, each one naming a dependency. As a 
shortcut, if there is only one dependency, $deps may be a string.

