template base directory

[bablukid]

allow to specify the template base directory. It's usefull with ::use:: to avoid to specify the full path to the file.

For instance this use case for a web page like described in https://github.com/ncannasse/templo

::use 'design.mtt'::
    <h2>The content to write in design.mtt</h2>
::end::

[Simn]

I'm not sure if this is really necessary. You can address the issue by pre-loading all used templates.

If we add this, it should at least deal with absolute vs. relative paths and make sure we don't run into problems with missing/additional slashes/backslashes. The haxe.io.Path API provides some functions for that like isAbsolute and join.