Add option to disable quickfix mappings

[atn34]

Adding quickfix mappings without an option to disable them makes this plugin interact poorly with plugins like https://github.com/stefandtw/quickfix-reflector.vim.

[mhinz]

The mappings got removed in favour of dedicated plugins.

[daGrevis]

Ohh, this broke my setup.

Here's how to restore it to something really similar:

1. Install https://github.com/romainl/vim-qf,
2. Enable mappings with let g:qf_mapping_ack_style = 1;