imp(Completions): adds fallbacks to Bash completions

With these options, in case the completion function cannot provide
suggestions, Bash will perform its default completions, based on e.g.
files, directories, and variable names. This is particularly useful for
argument values.

src/completions/bash.rs

@@ -58,7 +58,7 @@ impl<'a, 'b> BashGen<'a, 'b> {
     esac
 }}
 
-complete -F _{name} {name}
+complete -F _{name} -o bashdefault -o default {name}
 ",
                    name = self.p.meta.bin_name.as_ref().unwrap(),
                    name_opts = self.all_options_for_path(self.p.meta.bin_name.as_ref().unwrap()),

tests/completions.rs

@@ -21,5 +21,5 @@ fn test_generation() {
     let last_line = string.lines().rev().nth(0).unwrap();
 
     assert_eq!(first_line, "_myapp() {");
-    assert_eq!(last_line, "complete -F _myapp myapp");
+    assert_eq!(last_line, "complete -F _myapp -o bashdefault -o default myapp");
 }