Remove references to the old `open-term-by-default` var

[hkupty]

Additionally, add a variable for controlling new tabs sepparately from new windows.

This should fix #25

The new way of setting a new tab to open a term is by setting:

lua << EOF
local nvimux = require("nvimux")

nvimux.config.set_all{
  -- ...
  new_tab = "term",
  -- ...
}

-- ...

nvimux.bootstrap()