Add parameter `all` for --voices to show all voices

Update documentation to show already existing features and for
newly introduced `all` parameter.

src/espeak-ng.1.ronn

@@ -122,6 +122,13 @@ words are provided then text is spoken from stdin a line at a time.
   * `--voices[=<language code>]`:
     Lists the available voices. If =&lt;language code&gt; is present then only
     those voices which are suitable for that language are listed.
+    If `xx-yy` language code is passed, then voices with `yy` of `xx` language variants
+    are shown with higher priority than just `xx`.
+    If `variant` is passed, then all voice variants are shown.
+    If `mb` or `mbrola` is passed, then all voices using the MBROLA voice synthesizer
+    are shown.
+    If `all` is passed, then all eSpeak NG voices, voice variants and MBROLA
+    voices are shown.
 
   * `--voices=<directory>`:
     Lists the voices in the specified subdirectory.

src/libespeak-ng/voices.c

@@ -1184,6 +1184,11 @@ static int SetVoiceScores(espeak_VOICE *voice_select, espeak_VOICE **voices, int
 		if (((control & 1) == 0) && (memcmp(vp->identifier, "mb/", 3) == 0))
 			continue;
 
+		if (memcmp(voice_select->languages,"all", 3) == 0) {
+			voices[nv++] = vp;
+			continue;
+		}
+
 		if ((score = ScoreVoice(voice_select, language, n_parts, lang_len, voices_list[ix])) > 0) {
 			voices[nv++] = vp;
 			vp->score = score;