Fix template editor encoding problem

I'm sorry my poor English.


If encoding of document isn't ascii, current version of template editor generate this exception.

      File "/.../flask_debugtoolbar/panels/template.py", line 92, in template_editor
        for t in templates
    UnicodeDecodeError: 'ascii' codec can't decode byte 0xec in position 1473: ordinal not in range(128)

I fixed this bug, and add a new config variables:
DEBUG_TB_TEMPLATE_EDITOR_ENCODING - default is utf-8. if modify this variable, template editor recognize file's encoding.