parser: Try loading a parser from a plugin in case no parser is found

parser: Try loading a parser from a plugin in case no parser is found

When a parser is queried and one is not found within the ones that we
have already registered, try loading a plugin with that name. If
parser plugins are available, the parser might be available after the
plugin has been loaded.

The project is now fully compatible with having parsers as plugins.

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>