Added two possibilities to expand the index name

[anli-xsigns]

In a recent project I had the challenge to enable a better logging. Therefore I added two options to winston-elasticsearch:

Option 1: Enable to use a function for option property indexPrefix to use a variable name for the log.
Option 2: Introduced the indexInterfix property to be used in the log transformer to create individual logs for each log entry like prefix-interfix-suffix.

[coveralls]

Coverage decreased (-0.5%) to 81.69% when pulling 677addf on litti:master into 4bf2a5d on vanthome:master.

[vanthome]

lgtm but where is the documentation?

[litti]

I can write it if you want to accept the pr :-) If you tend to reject it I can save my time to write the docs :-)

[vanthome]

as I said, LGTM. I just would like to see documentation and that by default (if I don't use any of the new options) the behaviour remains as it is.

[vanthome]

thx! I just accepted that other PR, would you mind rebasing yours onto master?

[litti]

No problem, but don't know how to do. Just forked, made the changes, pushed them and made the pr.

[vanthome]

Google is your friend: https://github.com/edx/edx-platform/wiki/How-to-Rebase-a-Pull-Request

[litti]

of course, I've googled, but it all needs a lot of time spending for this and I'm close to holiday with tons of work to do so the one thing I don't have is time ;-). So never mind, I'll delete the pr so it doesn't mix up things here.

[litti]

Managed to rebase today.