Add es package

Looking for some feedback on this approach to talking to ES.
CatIndices is a slightly odd name here. Is "Cat" short for "Catalog" or is it like the unix command "cat" ?

How about just "AllIndexes" as a name? (indexes is the plural used
by elastic search itself and a quick google seems to indicate that
it's fine)

I'd suggest a doc comment like this:

    // AllIndexes returns the names of all the indexes in the database.

rather than mentioning the implementation details.