add options to set maximum download rate

- -r/--limit-rate as cmdline option
- downloader.http.rate as config option

This implementation very roughly uses the idea of the token bucket
algorithm [1] and mostly uses Wget's approach [2] as inspiration.

[1] https://en.wikipedia.org/wiki/Token_bucket
[2] http://git.savannah.gnu.org/cgit/wget.git/tree/src/retr.c?h=v1.19.2&id=ba6b44f6745b14dce414761a8e4b35d31b176bba#n111

docs/configuration.rst

@@ -120,6 +120,20 @@ Description Alternate location for ``.part`` files.
 =========== =====
 
 
+downloader.http.rate
+--------------------
+=========== =====
+Type        ``string``
+Default     ``null``
+Examples    ``"32000"``, ``"500k"``, ``"2.5M"``
+Description Maximum download rate in bytes per second.
+
+            Possible values are valid integer or floating-point numbers
+            optionally followed by one of ``k``, ``m``. ``g``, ``t`` or ``p``.
+            These suffixes are case-insensitive.
+=========== =====
+
+
 downloader.http.retries
 -----------------------
 =========== =====

docs/gallery-dl.conf

@@ -7,6 +7,7 @@
         "part-directory": null,
         "http":
         {
+            "rate": null,
             "retries": 5,
             "timeout": 30,
             "verify": true