feat(did-you-mean): for long flags (i.e. --long)

Long arguments now have a special case when saying they are unknown, as
we will match it against all known long flags and suggest the best match
to be used instead.

TODO: refactor to just write a suffix with did-you-mean information.

Related to #103

clap-tests/run_tests.py

@@ -40,6 +40,11 @@ USAGE:
 \tclaptests [POSITIONAL] [FLAGS] [OPTIONS] [SUBCOMMANDS]
 For more information try --help'''
 
+_arg_dym_usage = '''The argument --optio is unknown. Did you mean --option ?
+USAGE:
+\tclaptests [POSITIONAL] [FLAGS] [OPTIONS] [SUBCOMMANDS]
+For more information try --help'''
+
 _excluded = '''The argument '--flag' cannot be used with '-F'
 USAGE:
 \tclaptests [positional2] -F --long-option-2 <option2>
@@ -218,6 +223,7 @@ cmds = {'help short:         ': ['{} -h'.format(_bin), _help],
 		'mult_valsmo x2-1:   ': ['{} --multvalsmo some other --multvalsmo some'.format(_bin), _mult_vals_2m1],
 		'mult_valsmo x1:     ': ['{} --multvalsmo some other'.format(_bin), _exact],
 		'F2(ss),O(s),P:      ': ['{} value -f -f -o some'.format(_bin), _f2op],
+        'arg dym:            ': ['{} --optio=foo'.format(_bin), _arg_dym_usage],
 		'O2(ll)P:            ': ['{} value --option some --option other'.format(_bin), _o2p],
 		'O2(l=l=)P:          ': ['{} value --option=some --option=other'.format(_bin), _o2p],
 		'O2(ss)P:            ': ['{} value -o some -o other'.format(_bin), _o2p],

src/app.rs

@@ -1788,12 +1788,27 @@ impl<'a, 'v, 'ab, 'u, 'h, 'ar> App<'a, 'v, 'ab, 'u, 'h, 'ar>{
             return None;
         }
 
-        // Shouldn't reach here
+        match did_you_mean(arg, self.opts.values()
+                                         .filter_map(|v| 
+                                                if let Some(l) = v.long {Some(l)} else {None}
+                                          )
+                                         .collect::<Vec<_>>().iter()) {
+            Some(candidate_flag) => {
+                self.report_error(format!("The argument --{} is unknown. Did you mean --{} ?", 
+                                                                                arg, 
+                                                                                candidate_flag),
+                    true,
+                    true,
+                    None);
+            },
+            None => {
                 self.report_error(format!("The argument --{} isn't valid", arg),
                     true,
                     true,
                     Some(matches.args.keys().map(|k| *k).collect::<Vec<_>>()));
-        // Can't reach here...
+            }
+        }
+
         unreachable!();
     }