Enable formatters to only handle files with certain prefixes

A formatter can now be configured to only handle files that start with
a defined prefix. This is useful if files with the same mime type or
file extension in different folders should be handled by different
formatters. E.g.

  [formatter "ASCIIDOCTOR"]
    prefix = Documentation/
    mimeType = text/plain
  [formatter "MARKDOWN"]
    prefix = ReleaseNotes/
    mimeType = text/plain

Change-Id: If9af7606547dc13976773fa0ca36ba28b1ad21bd
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>