Set environment variables as configuration variables

Allows to refer env vars inside the configuration, as follows:

   background = "$XDG_CONFIG_HOME/wallpapers/file.png"

share/man/man1/hikari.md

@@ -160,6 +160,17 @@ terminal application.
 On startup **hikari** attempts to execute _~/.config/hikari/autostart_ to
 autostart applications.
 
+Environment Variables
+---------------------
+
+**hikari** supports the use of environment variables in its string values.
+Occurrences of `${VARIABLE}` or `$VARIABLE` inside a string will be substituted
+with the value of an environment variable named `VARIABLE`. If no such
+environment variable exists, no substituation takes place.
+
+You can use double dollar signs to escape variables: `$${VARIABLE}` will result
+in `${VARIABLE}`, `$$VARIABLE` will result in `$VARIABLE`.
+
 ACTIONS
 =======
 

src/configuration.c

@@ -36,6 +36,8 @@
 #include <hikari/view_config.h>
 #include <hikari/workspace.h>
 
+extern char **environ;
+
 struct hikari_configuration *hikari_configuration = NULL;
 
 static bool
@@ -1563,11 +1565,39 @@ done:
   return success;
 }
 
+bool
+set_env_vars(struct ucl_parser *parser)
+{
+  for (char **current_var = environ; *current_var != NULL; ++current_var) {
+    const char *separator = strchr(*current_var, '=');
+    if (separator == NULL) {
+      continue;
+    }
+
+    const size_t name_length = separator - *current_var;
+    char *name = hikari_malloc(name_length + 1);
+    if (name == NULL) {
+      fprintf(stderr, "Could not allocate enough memory :(\n");
+      return false;
+    }
+    strncpy(name, *current_var, name_length);
+    name[name_length] = '\0';
+    ucl_parser_register_variable(parser, name, separator + 1);
+    hikari_free(name);
+  }
+
+  return true;
+}
+
 bool
 hikari_configuration_load(
     struct hikari_configuration *configuration, char *config_path)
 {
   struct ucl_parser *parser = ucl_parser_new(0);
+  if (!set_env_vars(parser)) {
+    ucl_parser_free(parser);
+    return false;
+  }
   bool success = false;
   const ucl_object_t *cur;