Add option to create svg image with wordcloud_cli

[varac]

Description

I'd like to create an svg image with wordcloud_cli, which is currently not possible.

Steps/Code to Reproduce

--imagefile only references PNG image:

❯ wordcloud_cli --help | grep imagefile
                     [--imagefile file] [--fontfile path] [--mask file]
  --imagefile file      file the completed PNG image should be written to

Expected Results

wordcloud_cli should produce images based on the file extension. i.e. if I use --imagefile img.svg, a SVG should get produced.

Actual Results

❯ wordcloud_cli --imagefile img.svg --mask examples/alice_mask.png --text examples/constitution.txt
❯ file img.svg
img.svg: PNG image data, 900 x 900, 8-bit/color RGB, non-interlaced

Versions

❯ python /tmp/f.py 
Linux-5.19.0-76051900-generic-x86_64-with-glibc2.35
Python 3.10.6 (main, Aug 10 2022, 11:40:04) [GCC 11.3.0]
NumPy 1.23.4
matplotlib 3.6.1
Traceback (most recent call last):
  File "/tmp/f.py", line 5, in <module>
    import wordcloud; print("wordcoud", wordcloud.__version__)
ModuleNotFoundError: No module named 'wordcloud'

(I cloned main as of today)

[amueller]

I agree, that would be a great feature to have. Pull request welcome!