fix(did-you-mean): for review

* unknown subcommand message altered to use similar language as is used
  everywhere around clap. Namely, we say 'invalid' instead of 'unknown'
* 'did-you-mean' message separator changed from '. ' to '\n\t'

Related to #103

clap-tests/run_tests.py

@@ -35,18 +35,21 @@
     help      Prints this message
     subcmd    tests subcommands'''
 
-_sc_dym_usage = '''Subcommand "subcm" is unknown. Did you mean "subcmd" ?
+_sc_dym_usage = '''Subcommand "subcm" isn't valid
+\tDid you mean "subcmd" ?
 USAGE:
 \tclaptests [POSITIONAL] [FLAGS] [OPTIONS] [SUBCOMMANDS]
 For more information try --help'''
 
-_arg_dym_usage = '''The argument --optio isn't valid. Did you mean --option ?
+_arg_dym_usage = '''The argument --optio isn't valid
+\tDid you mean --option ?
 USAGE:
 \tclaptests
 For more information try --help'''
 
 _pv_dym_usage = '''"slo" isn't a valid value for '--Option <option3>'
-\t[valid values: fast slow]. Did you mean "slow" ?
+\t[valid values: fast slow]
+\tDid you mean "slow" ?
 USAGE:
 \tclaptests --Option <option3>
 For more information try --help'''

src/app.rs

@@ -1231,7 +1231,7 @@ impl<'a, 'v, 'ab, 'u, 'h, 'ar> App<'a, 'v, 'ab, 'u, 'h, 'ar>{
                                                                     I: IntoIterator<Item=&'z T> {
         match did_you_mean(arg, values) {
                 Some(candidate) => {
-                    let mut suffix = ". Did you mean ".to_string();
+                    let mut suffix = "\n\tDid you mean ".to_string();
                     match style {
                         DidYouMeanMessageStyle::LongFlag => suffix.push_str("--"),
                         DidYouMeanMessageStyle::EnumValue => suffix.push('"'),
@@ -1371,7 +1371,7 @@ impl<'a, 'v, 'ab, 'u, 'h, 'ar> App<'a, 'v, 'ab, 'u, 'h, 'ar>{
 
                 if let Some(candidate_subcommand) = did_you_mean(&arg, self.subcommands.keys()) {
                     self.report_error(
-                        format!("Subcommand \"{}\" is unknown. Did you mean \"{}\" ?",
+                        format!("Subcommand \"{}\" isn't valid\n\tDid you mean \"{}\" ?",
                             arg,
                             candidate_subcommand),
                         true,