Added possibility for SEO title

[lsolesen]

This is an enhancement or feature.

Summary

This makes it possible to add a SEO optimized title to the page like it is possible to have SEO optimized description. Related to #2371

Context

[iBug]

I'm afraid you didn't understand my comments on title_separator. In short, you need to keep it intact for existing users, i.e., users shouldn't be affected by your changes if they're relying on the current implementation.

[lsolesen]

@iBug You are completely right. Something went wrong. Please review again.

[lsolesen]

@mmistakes Did you think more about inclusion of this?

[lsolesen]

@mmistakes @iBug Did you think more about this.

[iBug]

I have no problem with this now, though I'm unconfident about @mmistakes 's views on this.

[lsolesen]

How do one avoid that pull requests does not go stale?

[iBug]

Now that seo.html has gone through some non-trivial changes, please re-implement this on top of current master and open a new PR - hopefully it'll be much easier to do so.

I'll be happy to review and merge when it comes up, just make sure to ping me.