Configure docutils through plugin configuration

[uyar]

This PR lets the user adjust docutils parameters. In the lektor project hierarchy, create a configs directory and place an rst.ini file in there. The docutils section of the file will be used to control docutils output. For example:

[docutils]
writer = html5
initial_header_level = 1

[fschulze]

I still have to try it, but so far this looks good.

[uyar]

I'm worried about the types of the option values. I couldn't be sure whether the types are converted automatically. For example, if there's an option with the value "yes", does it become the string "yes" or the boolean value True?

[uyar]

I'm closing this PR in order to submit it again from a new fork.