Make fuzzy completion customizable with _fzf_compgen_{path,dir}

Notes:
- You can now override _fzf_compgen_path and _fzf_compgen_dir functions
  to use custom commands such as ag instead of find for listing
  completion candidates.
    - The first argument is the base path to start traversal
- Removed file-only completion in bash, i.e. _fzf_file_completion.
  Maintaining a list of commands that only expect files, not
  directories, is cumbersome (there are too many) and error-prone.

TBD:
- Added $FZF_COMPLETION_DIR_COMMANDS to customize the list of commands
  which use directory-only completion. The default is "cd pushd rmdir".
  Not sure if it's the best approach to address the requirement, I'll
  leave it as an undocumented feature.

Related: #406 (@thomcom), #456 (@frizinak)

test/test_go.rb

@@ -1269,7 +1269,7 @@ module CompletionTest
     tmux.send_keys 'C-u'
     tmux.send_keys 'cat /tmp/fzf\ test/**', :Tab, pane: 0
     tmux.until(1) { |lines| lines.item_count > 0 }
-    tmux.send_keys :Enter
+    tmux.send_keys 'C-K', :Enter
     tmux.until do |lines|
       tmux.send_keys 'C-L'
       lines[-1].end_with?('/tmp/fzf\ test/foobar')
@@ -1339,6 +1339,20 @@ module CompletionTest
       tmux.send_keys 'C-L'
       lines[-1] == "kill #{pid}"
     end
+
+    def test_custom_completion
+      tmux.send_keys '_fzf_compgen_path() { echo "\$1"; seq 10; }', :Enter
+      tmux.prepare
+      tmux.send_keys 'ls /tmp/**', :Tab, pane: 0
+      tmux.until(1) { |lines| lines.item_count == 11 }
+      tmux.send_keys :BTab, :BTab, :BTab
+      tmux.until(1) { |lines| lines[-2].include? '(3)' }
+      tmux.send_keys :Enter
+      tmux.until do |lines|
+        tmux.send_keys 'C-L'
+        lines[-1] == "ls /tmp 1 2"
+      end
+    end
   ensure
     Process.kill 'KILL', pid.to_i rescue nil if pid
   end