Allow themes to be disabled in rline

This still outputs ANSI color escapes, they're just always for color 9,
which is the default color for the foreground or background. rline will
always output escapes, as it's fundamentally part of how it works. If
you don't want ANSI escapes, disable rline.

src/vendor/rline.c

@@ -363,19 +363,19 @@ static void recalculate_tabs(line_t * line) {
  */
 static const char * COLOR_FG        = "@9";
 static const char * COLOR_BG        = "@9";
-static const char * COLOR_ALT_FG    = "@5";
+static const char * COLOR_ALT_FG    = "@9";
 static const char * COLOR_ALT_BG    = "@9";
-static const char * COLOR_KEYWORD   = "@4";
-static const char * COLOR_STRING    = "@2";
-static const char * COLOR_COMMENT   = "@5";
-static const char * COLOR_TYPE      = "@3";
-static const char * COLOR_PRAGMA    = "@1";
-static const char * COLOR_NUMERAL   = "@1";
-static const char * COLOR_RED       = "@1";
-static const char * COLOR_GREEN     = "@2";
-static const char * COLOR_ESCAPE    = "@2";
-static const char * COLOR_SEARCH_FG = "@0";
-static const char * COLOR_SEARCH_BG = "@3";
+static const char * COLOR_KEYWORD   = "@9";
+static const char * COLOR_STRING    = "@9";
+static const char * COLOR_COMMENT   = "@9";
+static const char * COLOR_TYPE      = "@9";
+static const char * COLOR_PRAGMA    = "@9";
+static const char * COLOR_NUMERAL   = "@9";
+static const char * COLOR_RED       = "@9";
+static const char * COLOR_GREEN     = "@9";
+static const char * COLOR_ESCAPE    = "@9";
+static const char * COLOR_SEARCH_FG = "@9";
+static const char * COLOR_SEARCH_BG = "@9";
 static const char * COLOR_ERROR_FG  = "@9";
 static const char * COLOR_ERROR_BG  = "@9";
 static const char * COLOR_BOLD      = "@9";
@@ -2439,8 +2439,12 @@ int rline(char * buffer, int buf_size) {
 	offset = 0;
 	buf_size_max = buf_size;
 
+	char * noColor = getenv("NO_COLOR");
 	char * theme = getenv("RLINE_THEME");
-	if (theme && !strcmp(theme,"sunsmoke")) { /* TODO bring back theme tables */
+	if ((noColor && *noColor) || (theme && !strcmp(theme,"none"))) {
+		/* If NO_COLOR is set, or RLINE_THEME=none, set no theme.
+		 * The default colors are all @9. */
+	} else if (theme && !strcmp(theme,"sunsmoke")) {
 		rline_exp_load_colorscheme_sunsmoke();
 	} else {
 		rline_exp_load_colorscheme_default();