Builtin prefix support

[advilm]

Currently, prefixes are defined on a per plugin basis in a way that when you type a prefix it allows that plugin to run. However, every other plugin can still show up as well (especially ones that do not require a prefix). When I type a prefix, it usually means I want to only use a specific plugin. It would be nice to have a way to only run a specific plugin when I use a specific prefix.

I think to implement this, it would be necessary to actually have a builtin prefix feature. Each plugin could optionally specify a prefix through PluginInfo. If a prefix is specified, then anyrun can know when to only run that plugin. This would also allow the removal of all the duplicate code for handling prefix in each of the existing plugins.

The main downside of implementing this is that anyone who is using a custom plugin will have to make a minor modification to specify that the plugin doesn't set a prefix.

Is implementing something like this reasonable for this project? If so, I'd like to try implementing it myself.

[Kirottu]

The problem with providing it in PluginInfo with the current implementation is that it would need to load the plugin config twice, first for the plugin info and second while doing the initialization. That could probably be worked around by redesigning the API a bit.

Regardless, if you want to tinker around with it and try to figure out if it would be feasible, I would recommend tinkering with the anyrun-provider-rebase branch as seen in #251, since that already has a large amount of changes to the plugin system and will be merged into master soon enough.