(Config) Introduce `input.charset` and read files with it

[OndraZizka]

JBake only has render.encoding, which is what the documents are both read and written.

The source files however may have different encoding than the result. I have a bunch of old files that are windows-1250, but want the site to be rendered to utf-8.

This, together with #490 , will save me from converting the old files and checking if anything went wrong.

I have a fix and will give a PR.

[jonbullock]

I'm guess you need #490 to allow you to set the value of input.charset differently for certain directories?