Adding option to choose default result window

[patrickwhite256]

• Adding g:ag_default_window_type to choose to use the quickfix or location list
• Adding :LAg... variants of commands that did not already have them
• Adding :QAg... variants of all commands

Previously, it was difficult to use the location list by default, since something like command -nargs=* Ag LAg <args> in the vimrc would get overridden by the command! in the plugin.

In my opinion, the variants should eventually be removed, since I don't really see a reason why someone would want to use one list sometimes but the other sometimes. If people agree with me:

• The :LAg... variants should be put on a deprecation path
• The :QAg... variants should just be removed now

If you agree they should be removed, I'll update the PR to do these things.