add content for prev- and next- links in css

[manumaticx]

This adds support for customizing those links.
Closes #151

[coveralls]

Coverage remained the same at 82.842% when pulling 70e76d5 on manumaticx:issue-151 into c19f214 on react-component:master.

[x5engine]

this is still not fixed! the next and previous are empty!

I had to add

:after {
content: '‹'
}