userscripts: Add QUTE_VERSION

See #937

doc/changelog.asciidoc

@@ -116,6 +116,8 @@ Added
   via a new `filesystem` entry in `completion.open_categories`. Also, a new
   `completion.favorite_paths` setting was added which can be used to add paths to
   show when `:open` is used without any input.
+- New `QUTE_VERSION` variable for userscripts, which can be used to read
+  qutebrowser's version.
 - New userscripts:
   * `kodi` to play videos in Kodi
   * `qr` to generate a QR code of the current URL

doc/userscripts.asciidoc

@@ -43,6 +43,9 @@ The following environment variables will be set when a userscript is launched:
   qutebrowser is still in command mode. If you want to receive arguments passed
   to your userscript via `:spawn`, use the normal way of getting commandline
   arguments (e.g. `$@` in bash or `sys.argv` / `argparse` / ... in Python).
+- `QUTE_VERSION`: The version of qutebrowser, as a string like "2.0.0". Note that this
+  was added in v2.0.0, thus older versions can only be detected by the absence of this
+  variable.
 
 In `command` mode:
 

qutebrowser/commands/userscripts.py

@@ -26,6 +26,7 @@ from typing import cast, Any, MutableMapping, Tuple
 
 from PyQt5.QtCore import pyqtSignal, pyqtSlot, QObject, QSocketNotifier
 
+import qutebrowser
 from qutebrowser.utils import message, log, objreg, standarddir, utils
 from qutebrowser.commands import runners
 from qutebrowser.config import websettings
@@ -444,6 +445,7 @@ def run_async(tab, cmd, *args, win_id, env, verbose=False,
     env['QUTE_DOWNLOAD_DIR'] = downloads.download_dir()
     env['QUTE_COMMANDLINE_TEXT'] = objreg.get('status-command', scope='window',
                                               window=win_id).text()
+    env['QUTE_VERSION'] = qutebrowser.__version__
 
     cmd_path = os.path.expanduser(cmd)