add greasemonkey support

README.md

@@ -117,12 +117,34 @@ The lists are downloaded on first start and compiled into
 `:adblock-update` re-downloads them. `:adblock-toggle` switches blocking off and
 on, for when a site breaks.
 
+## Greasemonkey
+
+Userscripts go in `~/.local/share/att/att/greasemonkey/` as `*.js` files. They
+are loaded at startup; after changing them, run `:greasemonkey-reload` and reload
+the page. Like qutebrowser, att:
+
+- runs scripts in the page's own JavaScript context (so `unsafeWindow` is just
+  `window`), in iframes too unless `@noframes` is set;
+- honours `@include`, `@exclude` and `@match` (checked by QtWebEngine itself; a
+  script with none of them runs everywhere), `@run-at document-start` /
+  `document-end` (the default) / `document-idle`, and `@require`, which is
+  downloaded once into `greasemonkey/requires/`;
+- provides `GM_info`, `GM_setValue`/`GM_getValue`/`GM_deleteValue`/`GM_listValues`,
+  `GM_addStyle`, `GM_addElement`, `GM_xmlhttpRequest`, `GM_openInTab`,
+  `GM_setClipboard`, `GM_log`, `GM_notification` and the promise-based `GM.*`
+  versions.
+
+Limits: stored values live in the site's `localStorage`, so they are per site
+rather than shared across sites. `GM_xmlhttpRequest` is a plain XHR, so it can't
+make cross-origin requests the server doesn't allow (CORS). `@resource` isn't
+supported. `GM_openInTab` does nothing until new-window requests are handled.
+
 ## Commands
 
 `:open [-w] <url|query>` (alias `:o`; `-w`, or qutebrowser's `-t`, opens a new
 window), `:close` (alias `:tab-close`, closes this window), `:quit` (alias `:q`,
 closes all windows), `:reload`, `:back`, `:forward`, `:stop`, `:history-clear`,
-`:adblock-update`, `:adblock-toggle`.
+`:greasemonkey-reload`, `:adblock-update`, `:adblock-toggle`.
 
 `:open` accepts a full URL, a bare host (`example.com` → `https://example.com`),
 an existing local path, or free text, which goes to DuckDuckGo.
@@ -136,6 +158,7 @@ an existing local path, or free text, which goes to DuckDuckGo.
 | `src/main.cpp`      | `QApplication`, CLI parsing, stylesheet                 |
 | `src/browserwindow` | the window, mode state machine, key dispatch, commands  |
 | `src/commandline`   | the `:` prompt                                          |
+| `src/greasemonkey`  | userscript loading and the GM_* API wrapper             |
 | `src/history`       | SQLite browsing history                                 |
 | `src/completionview`| the `:open` completion list                             |
 | `src/statusbar`     | mode / URL / progress line                              |