Current circumstance

Inside the title parameter, defined in the Front Matter, some users can use html tags, for example <br>, <em> or <strong>.
Example of use: Tag in title.

Problem

Open Graph does not support html tags nests inside the og:title property:

Proposal

Adding plainify to clean up these tags will prevent the title from being rendered inappropriately when sharing a page on Facebook for example.

The code of the solution I suggest is in this PR: 68c69c8