Add `import` function.

This commit adds an global `import` function which can be used to import
shit from other files.

Example of it's usage:

    <%
        stuff_from_other_file = import("some_other_file.tly");

        response.write(stuff_from_other_file["variable_from_other_file"]);
    %>

Files can only be imported once. The resulting hash map is cached into
the interpreter and that cached version is returned on subsequent import
calls of the same file.
Merge pull request #57 from peelonet/import-function

Add `import` function.