config+url: add support for user-defined regex patterns

Users can now define their own regex patterns, and use them via key
bindings:

    [regex:foo]
    regex=foo(bar)?
    launch=path-to-script-or-application {match}

    [key-bindings]
    regex-launch=[foo] Control+Shift+q
    regex-copy=[foo] Control+Mod1+Shift+q

That is, add a section called 'regex:', followed by an
identifier. Define a regex and a launcher command line.

Add a key-binding, regex-launch and/or regex-copy (similar to
show-urls-launch and show-urls-copy), and connect them to the regex
with the "[regex-name]" syntax (similar to how the pipe-* bindings
work).

config.h

@@ -61,6 +61,7 @@ enum binding_aux_type {
     BINDING_AUX_NONE,
     BINDING_AUX_PIPE,
     BINDING_AUX_TEXT,
+    BINDING_AUX_REGEX,
 };
 
 struct binding_aux {
@@ -74,6 +75,8 @@ struct binding_aux {
             uint8_t *data;
             size_t len;
         } text;
+
+        char *regex_name;
     };
 };
 
@@ -121,6 +124,13 @@ struct env_var {
 };
 typedef tll(struct env_var) env_var_list_t;
 
+struct custom_regex {
+    char *name;
+    char *regex;
+    regex_t preg;
+    struct config_spawn_template launch;
+};
+
 struct config {
     char *term;
     char *shell;
@@ -225,6 +235,8 @@ struct config {
         regex_t preg;
     } url;
 
+    tll(struct custom_regex) custom_regexes;
+
     struct {
         uint32_t fg;
         uint32_t bg;