Set default type in init

[samuelallan72]

It could be useful to set the default type ('start' or 'opt') in minpac#init to reduce verbosity if a majority of added packages would to be of type 'opt'? Eg.

call minpac#init({'package_name': 'bundle'})
call minpac#add('k-takata/minpac', {'type': 'opt'})

Would be the same as:

call minpac#init({'package_name': 'bundle', 'default_type': 'opt'})
call minpac#add('k-takata/minpac')