Replace octicon element with a static unicode symbol

[N7K4]

There was a octicon span element, this is now replaced by the configure able anchor_symbol unicode link symbol 🔗

The symbol is placed behind the headline, the octicon span was placed before the headline

Fixes #24

[domdfcoding]

I've tried this out and it appears to work as expected. I've rebased atop the latest version of jekyll-toc in my own fork: https://github.com/potbanksoftware/jekyll-toc/commits/my0150/

There were only a few minor merge conflicts but it works on my site.

[domdfcoding]

Actually I did run into one issue. With a header like this:

# Foo `bar`

the chain symbol gets inserted between the plain text "Foo" and the code tag for "bar".

I did 0283cad180e160c4ae499ae9d221060e177d62bf which appears to work but I do not know ruby and didn't write tests.