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

Fixes #32
fix typo
Merge pull request #54 from nicoder/patch-1

fix typo
Merge branch 'master' of https://github.com/rking/ag.vim

# By nicoder
# Via Josh Hoff (1) and nicoder (1)
* 'master' of https://github.com/rking/ag.vim:
  fix typo
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 branch 'master' of https://github.com/rking/ag.vim

# By Joshua Hoff
# Via Josh Hoff (1) and Joshua Hoff (1)
* 'master' of https://github.com/rking/ag.vim:
  HOTFIX: check for ag executable using g:agprg
  Don't try to search if `ag` isn't available
Backup g:agformat when changing it

Otherwise running :AgFile followed by :Ag will lead to results that
can't be jumped to.
typo
Merge pull request #64 from fernandoacorreia/patch-1

typo
Merge pull request #60 from losingkeys/agfile-breaks-other-searches

Backup g:agformat when changing it
Merge branch 'master' of https://github.com/rking/ag.vim

# By Fernando Correia (1) and Joshua Hoff (1)
# Via Josh Hoff (2) and Fernando Correia (1)
* 'master' of https://github.com/rking/ag.vim:
  typo
  Backup g:agformat when changing it