Parse render filters from the config

Parse render filters from the config

Render filters will be used to present rendered content in the tree
view, for example to display Markdown source rendered as HTML.

We will add support for using these from the tree view in the following
commits.

AG: adapted so render.= can be used to specify the filter for files
without any suffix

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Andy Green <andy@warmcat.com>
Reviewed-by: John Keeping <john@keeping.me.uk>