Adds custom list of plugins to be passed on load

[CyclingNinja]

Adds customisable list of plugins on load

Adds an argument to load_plugins in main that allows a custom list to be passed.
On empty, defaults to required plugins which have been added to _plugin_helpers

[astrofrog]

This is looking good so far, but think we need to tweak the logic related to requiring plugins - see below. API-wise I think the keyword argument to load_plugins should probably just be called names so that it would look like:

load_plugins(names=['glue.core.something'])

rather than

load_plugins(plugins_to_load=['glue.core.something'])

which is a bit repetitive. Internally we could still use plugins_to_load though.

[astrofrog]

I think it might be best to actually define n_plugins as len(plugins_to_use) once plugins_to_use has been defined.

[astrofrog]

I don't think plugins_to_use should be based on the REQUIRED_* variables - these are just used to raise exceptions below if they are not present. I think we actually need two different variables inside this function - plugins_to_load(perhaps a better name than plugins_to_use) and plugins_to_require. I think the logic should be:

• If plugins_to_load is not defined by the user, then plugins_to_load defaults to all entry points (iter_plugin_entry_points) and plugins_to_require defaults to either REQUIRED_PLUGINS or the combination of REQUIRED_PLUGINS and REQUIRED_PLUGINS_QT
• If plugins_to_load is defined by the user, then plugins_to_require should just be set to plugins_to_load because if the user is explicitly asking for them we should error if they can't be loaded

[astrofrog]

I can't easily comment on lines above but based on my previous comment,

     if item.module in REQUIRED_PLUGINS:
                raise
            elif item.module in REQUIRED_PLUGINS_QT and require_qt_plugins:
                raise

should then become:

     if item.module in plugins_to_require:
                raise

[astrofrog]

Looks good, thanks! In a follow-up PR, could you tweak the API of load_plugins so that the kwarg is called names instead of plugins_to_load? (see above comment about this).