List available data sources

[jgbarah]

Right now, there is no way of knowing which data sources are available for generating a report. It would be great having something like:

$ manuscripts --list-data-sources
Available data sources: git gerrit github

At least on a first try, those could be just the data sources supported by Manuscripts, in general.

[f2014169]

@jgbarah by data sources u mean ES indexes??

[aswanipranjal]

@f2014169, data sources are from where data about the project is being retrieved and put into the indices. Data sources can be git to get the commit data, GitHub to get the issue data, mailing lists, gerrit, jira, stackexchange and so on.
Data Sources does not mean indices.