Don't try to search if `ag` isn't available

Fixes #32
Merge pull request #47 from losingkeys/check-for-ag-command

Don't try to search if `ag` isn't available
HOTFIX: check for ag executable using g:agprg

Thanks @Cemenyave via
https://github.com/losingkeys/ag.vim/commit/261807760f73f52a33d1571a5ca028917a8168c3#commitcomment-6406596
Merge remote-tracking branch 'upstream/master'