Add Template as Metatada enhancement proposal

jupyter-template-as-metadata/jupyter-template-as-metadata.md

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+# Jupyter Template as Metadata
+
+## Problem
+
+[nbconvert](http://nbconvert.readthedocs.io/) supports [custom templates](http://nbconvert.readthedocs.io/en/latest/customizing.html#Custom-Templates),
+but there is no way to save a custom template as the default template for a Jupyter Notebook,
+something that would help with the reproducibility of converted documents
+and facilitate the cases illustrate below.
+
+### Case 1: Internal Reports
+
+A team working at company X uses Jupyter Notebook at work
+and need to generate a weekly report.
+The reports **must** be branded with the company logo at the first page upper right corner.
+The report release staff whish to click on `File > Download as > PDF via LaTeX (.pdf)`
+and get the file they need to circulate
+**but** they need to run nbconvert from the command line.
+
+### Case 2: Meetup Supplement Material
+
+A data science [Meetup](https://www.meetup.com/) group meet weekly
+and each meeting someone present something
+and provide a Jupyter Notebook as supplement material.
+The group want to provide the supplement material at their website
+with a end note advertising the group.
+The group website master whish to click on `File > Download as > HTML (.html)`
+and the file with the end note
+**but** they need to run nbconvert from the command line.
+
+### Other cases
+
+Academic journals and conference proceedings are two other cases that could benefit of this enhancement.
+
+## Proposed Enhancement
+
+[Notebook metadata](http://nbformat.readthedocs.io/en/latest/format_description.html#notebook-metadata)
+will have a key `template` which value will be a dictionary
+to store the location of the default custom template to be used by nbconvert.
+
+## Detailed Explanation
+
+### Notebook Metadata
+
+`template` will be a defined notebook metadata key that will have as value something like
+
+~~~
+{
+  "html": "path/to/html/custom/template",
+  "md": "path/to/markdown/custom/template",
+  "rst": "path/to/rest/custom/template",
+  "pdf": "path/to/pdf/custom/template",
+  "py": "path/to/python/custom/template",
+  "r": "path/to/r/custom/template",
+  "jl": "path/to/julia/custom/template"
+}
+~~~  
+
+No key inside `template`-value is required.
+
+The path to the template can be
+
+- a full path (in Unix-like format) to the local filesystem
+- a relative path (in Unix-like format) to the local filesystem
+- a online address with the protocol specified
+
+### nbconvert behavior
+
+nbconvert will first try to use the custom template provided during the command line call
+following back to the custom template specified in the notebook metadata to be converted
+and as a last resource using the default template provided by the Jupyter project.
+
+### Jupyter Notebook Web Interface
+
+Jupyter Notebook will provide a option at menu bar
+for users set up the custom template they want to associate with the notebook.
+For example,
+
+![Screenshot suggestion](jupyter-notebook-menu.png)
+
+## Pros and Cons
+
+* Pro: Enable users to download the Notebook with a custom format
+* Pro: Make the custom template feature more visible
+* Con: Increase the number of options at the menu
+
+## Interested Contributors
+
+@rgaiacs