Allows files argument to be parsed to ag/search

[vitorqb]

Gives ag/search an optional files parameter. If parsed, give this
list of files to ag instad of the current directory (".").

I find this usefull because I use org-journal, and now I can do
something like this to search with ag between two dates, for example
2019-15-5->2019-05-31:

(let* ((start (org-journal-calendar-date->time '(5 15 2019)))
       (end   (org-journal-calendar-date->time '(5 31 2019)))
       (files (sort (org-journal-search-build-file-list start end) #'string<)))
  (ag/search "foo" "~/Documents/journal" :files files))

[coveralls]

Coverage remained the same at 18.773% when pulling 609d047 on vitorqb:allow-file-list-on-ag-search into 77b4f50 on Wilfred:master.

[Wilfred]

I don't mind adding an extra keyword parameter to ag/search, but why have you added an extra -H argument to ag?

[vitorqb]

Sorry, my mistake. For some reason i thought it would only working with the -H flag, but I did some tests now and I think I was wrong.

[Wilfred]

Thanks :)