Add a setting for default metadata for contents

[lebouquetin]

Currently, users need to define all metadata even the repetitive ones.

Expected: allow to set a default value for a series of metadata.

A way to do so would be to define a default.metada text file with default content. At load time, this file is concatenated as the first lines of metadata so that:

• default values are available
• default values may be overwritten by value defined in the content itself.

Example:

Currently, we can overwritte a metadata:

--- METADATA (first) ---
title   galae - le service e-mail éthique et libre facturé à l'usage
slug    fr-index
slug    de-index

Will result with a slug metadata equal to de-index

So the behaviour would be:

default.metada file content:

lang            fr
template_engine jinja

And a content file:

--- METADATA (first) ---
title   galae - le service e-mail éthique et libre facturé à l'usage
slug    fr-index
slug    de-index

would result in a content equivalent to:

--- METADATA (first) ---
lang            fr
template_engine jinja
title           galae - le service e-mail éthique et libre facturé à l'usage
slug            de-index

the settings.py configuration file would contain 2 new parameters:

• JFME_DEFAULT_METADATA_FILEPATH = <path> which links to the file on the disk. The file is a text file without any header: only lines of metadata <metadata_name> <metadata_value>
• JFME_DEFAULT_METADATA_STRING = "" which gives direct access to configuring default metadata in settings.py.

The order of include would be:

1. read metadata from setting.py found in JFME_DEFAULT_METADATA_STRING
2. read metadata from JFME_DEFAULT_METADATA_FILEPATH
3. read from content file itself

[ClmntBcqt]

JFME_DEFAULT_METADATA_STRING is replaced by JFME_DEFAULT_METADATA_DICT which is a dictionary, for a better format in a python file.
Example : JFME_DEFAULT_METADATA_DICT = {"slug": "index", "title": "Default title", "lang": "en"}

[ClmntBcqt]

Merged in #40