Make it possible to create erlydtl beam files with full path in the name

if DTL_FULL_PATH=true
  /templates/a/b/c.dtl -> a_b_c_dtl.beam
otherwise (as it was before)
  -> c_dtl.beam

add return_errors flag to erlydtl:compile to return {error, Errors} instead of error;
add {doc_root, "templates"} flag to allow extend and include templates from parent directories (by default erlydtl looks for parent template in the directory of the current template);
make compilation stop if erlydtl:compile returns an error