plugin-loader: Add optional filter for plugin features

plugin-loader: Add optional filter for plugin features

In some cases, the algorithms that have been compiled into a plugin have
to be disabled at runtime. Based on the array returned by the get_features()
function the optionally provided function can strip algorithms or even
callbacks or registrations from a plugin, giving us a handy and powerful way
for runtime feature configuration aside from the plugin list.

Signed-off-by: Thomas Egerer <thomas.egerer@secunet.com>