fix: do not override user-specified warnings filter

Currently the Lektor CLI unconditionally sets the warnings filter to
"default". There is no way of the user to configure this.

Here we bypass re-configuring the warnings filter if it has already
been customized, either via the PYTHONWARNINGS environment variable or
by python's -W flag.

E.g., now one can show all warnings by doing either of:

    PYTHONWARNINGS=always lektor build

or

    python -W always -m lektor build

Ref: https://docs.python.org/3/library/warnings.html#overriding-the-default-filter

lektor/cli.py

@@ -42,7 +42,8 @@ def cli(ctx, project=None, language=None):
     This command can invoke lektor locally and serve up the website.  It's
     intended for local development of websites.
     """
-    warnings.simplefilter("default")
+    if not sys.warnoptions:
+        warnings.simplefilter("default")
     if language is not None:
         ctx.ui_lang = language
     if project is not None: