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)

README.md

@@ -259,6 +259,14 @@ export FZF_COMPLETION_TRIGGER='~~'
 
 # Options to fzf command
 export FZF_COMPLETION_OPTS='+c -x'
+
+# Use ag instead of the default find command for listing candidates.
+# - The first argument to the function is the base path to start traversal
+# - Note that ag only lists files not directories
+# - See the source code (completion.{bash,zsh}) for the details.
+_fzf_compgen_paths() {
+  ag -g "" "$1"
+}
 ```
 
 #### Supported commands