Make calling require'lean'.setup{} idempotent

[Julian]

Users occasionally will put this in a file they reload or re-source, so it'd be nice if it were able to be run multiple times.

Right now most of the state issues are simply with initializing global configuration, so this may not be too difficult to do.

(See #132 for some context.)

Relates to #39.

[4e554c4c]

the solution in my config

local success, err = pcall(function ()
  require('lean').setup{}
end)
if not success then
  print(err)
end