#38 separated json into separate file

[jdaguil]

writing data.json file into the assets folder instead of into the template

[pombredanne]

@jdaguil can you elaborate why you updated the logic here?

[jdaguil]

@pombredanne The change was to make the check for '' more explicit.

BEFORE:

tgt_dirs = get_html_app_files_dirs(output_file)
if not tgt_dirs:
    raise HtmlAppAssetCopyWarning()         # not clear warning from stdout

AFTER:

if is_stdout(output_file):
    raise HtmlAppAssetCopyWarning()
tgt_dirs = get_html_app_files_dirs(output_file)         # output valid, no need to check