Do not map <Enter> everywhere

First, thanks for the plugin, I wanted to do something similar myself so you spare me to write it :P I use the `<Enter>` key myself for launching the tests but inside vim it has some inconvenience: for example if you use the [cmdwin]() the `<Enter>` key there is used for other purposes, the same thing is true for the Quickfix List and the Location List, plus other plugins like Tagbar, NERDTree, netwr etc etc. So I tried with something like this and it seems to work. I couldn't test this thing though because [vim-spec]() launch the tests with vim in Ex-mode so no windows operations can be used.