Allow colons in plain scalars inside flow collections

This is a followup to yaml#28

See http://yaml.org/spec/1.1/#nb-plain-char(c) and the following
productions.

This commit will allow `[http://example]`, but still fail for:
- `[:foo]`
- `[foo:]`

src/scanner.c

@@ -3434,7 +3434,15 @@ yaml_parser_scan_plain_scalar(yaml_parser_t *parser, yaml_token_t *token)
 
             if (parser->flow_level
                     && CHECK(parser->buffer, ':')
-                    && !IS_BLANKZ_AT(parser->buffer, 1)) {
+                    && (
+                        CHECK_AT(parser->buffer, ',', 1)
+                        || CHECK_AT(parser->buffer, '?', 1)
+                        || CHECK_AT(parser->buffer, '[', 1)
+                        || CHECK_AT(parser->buffer, ']', 1)
+                        || CHECK_AT(parser->buffer, '{', 1)
+                        || CHECK_AT(parser->buffer, '}', 1)
+                    )
+                    ) {
                 yaml_parser_set_scanner_error(parser, "while scanning a plain scalar",
                         start_mark, "found unexpected ':'");
                 goto error;
@@ -3444,7 +3452,7 @@ yaml_parser_scan_plain_scalar(yaml_parser_t *parser, yaml_token_t *token)
 
             if ((CHECK(parser->buffer, ':') && IS_BLANKZ_AT(parser->buffer, 1))
                     || (parser->flow_level &&
-                        (CHECK(parser->buffer, ',') || CHECK(parser->buffer, ':')
+                        (CHECK(parser->buffer, ',')
                          || CHECK(parser->buffer, '?') || CHECK(parser->buffer, '[')
                          || CHECK(parser->buffer, ']') || CHECK(parser->buffer, '{')
                          || CHECK(parser->buffer, '}'))))