Add option to use og:description as description

[hugovk]

For #65.

Adds a Boolean option for conf.py called enable_meta_description, which when true, will add the same description to <meta name="description" content="..."> as used for <meta property="og:description" content="...">.

We're looking into using this extension in the CPython docs (for example: official docs, devguide, PEPs), and improving the SEO by adding the meta description would be very helpful.

Re: python/docs-community#65

[TheTripleV]

Sorry, this was merged prematurely. It's been reverted for now.

A few changes required:

1. Config options should be preceded with ogp. I think ogp_enable_meta_description would be fine.
2. I'm not a fan of modifying the config (config["ogp_custom_meta_tags"]). I understand that this is a workaround because tags is a dict. We deferred running make_tags on purpose to make it easier to detect conflicting tags.
3. New config options should be added to the documentation.

I'll leave additional thoughts on the overall concept/design in #65.

[hugovk]

1. Sounds good.
2. Yes, that was replaced with another dict in Add support for meta description #72.
3. Good idea.

But let's continue in #65.