tokenize: handle escaped quotes

We only support escaping quotes inside a quote, and only of the same
type as the enclosing quote.

I.e.

  "double quote \"escaped double quote\""

But not

  "double quote \'invalid escaped single quote\'"

CHANGELOG.md

@@ -18,6 +18,8 @@
 * [LICENSE](LICENSE), [README.md](README.md) and
   [CHANGELOG.md](CHANGELOG.md) are now installed to
   `${datadir}/doc/foot`.
+* Support for escaping quotes in **pipe-visible** and
+  **pipe-scrollback** commands.
 
 
 ### Changed

tokenize.c

@@ -38,10 +38,11 @@ tokenize_cmdline(char *cmdline, char ***argv)
     char delim = first_token_is_quoted ? cmdline[0] : ' ';
 
     char *p = first_token_is_quoted ? &cmdline[1] : &cmdline[0];
+    char *search_start = p;
 
     size_t idx = 0;
     while (*p != '\0') {
-        char *end = strchr(p, delim);
+        char *end = strchr(search_start, delim);
         if (end == NULL) {
             if (delim != ' ') {
                 LOG_ERR("unterminated %s quote", delim == '"' ? "double" : "single");
@@ -58,6 +59,15 @@ tokenize_cmdline(char *cmdline, char ***argv)
                 return true;
         }
 
+        if (end > p && *(end - 1) == '\\') {
+            /* Escaped quote, remove one level of escaping and
+             * continue searching for "our" closing quote */
+            memmove(end - 1, end, strlen(end));
+            end[strlen(end) - 1] = '\0';
+            search_start = end;
+            continue;
+        }
+
         *end = '\0';
 
         if (!push_argv(argv, &argv_size, p, &idx))
@@ -75,6 +85,7 @@ tokenize_cmdline(char *cmdline, char ***argv)
             p++;
         } else
             delim = ' ';
+        search_start = p;
     }
 
     if (!push_argv(argv, &argv_size, NULL, &idx))