Added extractYaml option

[faisalraja]

Handles yaml in front of markdown.

[Ionaru]

Can you add the new extractYaml option to easymde.d.ts?

[faisalraja]

@Ionaru can you check issue #371 I have a question there, basically maybe it should always just be yaml-frontmatter and when extractYaml is provided, then it's going to remove it from preview? Right now it's also using extractYaml to decide if we'll use yaml-frontmatter.

[Ionaru]

I think maybe we need to go a step further and always remove the yaml-frontmatter from the preview? Regardless of any option. I don't think there's any situation where you want to see it.

Then a extractYaml function can do something with the data if it wants, otherwise it's just sent as-is to the server.

[faisalraja]

@Ionaru Makes sense. Updated.