Adds navigation for posts marked as a part of a series.

[koundinya]

Context -
http://www.joesacher.com/blog/2017/08/03/implementing-series-in-hugo/

Kiss has the ability to add related posts, but when you are writing a series of posts, it would be helpful to have a way to navigate between them.

Current implementation is based on the link mentioned above.

1. This can be made configurable from config.toml , would need changes in the pr.
2. Currently I've added
   {{ partial "series_navigation.html" . }}
   after the content it could be placed above as well.

[ribice]

Sorry for replying this late. I'd like to address this PR now.

What other changes are needed in config.toml from your side to get this merged?

[koundinya]

I was thinking it should be left to the user to decide if they need series navigation or not and that should be configured via the config.toml, wasn't sure if this is the direction you would like to take and didn't implement. In terms of code, {{ partial "series_navigation.html" . }} should have a check to see if the feature is enabled in the config. What are your thoughts on this?

[ribice]

I'd keep it at the bottom only for now and make it configurable in config.toml

How do you mark a post as being in series?

[ribice]

Is this PR ready @koundinya ?