Non-int margin value in html style attributes

Hi,

I got a value error(**ValueError: invalid literal for int() with base 10: 'au'**) while reading style string for a html tag. The attribute was margin-left and its value was auto instead of an integer/float.

```
Traceback (most recent call last):
File "C:\Users\keshav.bohra\PycharmProjects\git\content-comparison-advisor\minton\dashboard\views.py", line 98, in post
pages_count, html_files = comparePDFHTMLV2(INPUT_FILES)
File "C:\Users\keshav.bohra\PycharmProjects\git\content-comparison-advisor\minton\dashboard\file_compare\pdfhtmlcompare.py", line 306, in comparePDFHTMLV2
html_parser.parseV2(htmlPath)
File "C:\Users\keshav.bohra\PycharmProjects\git\content-comparison-advisor\minton\dashboard\file_compare\parser.py", line 78, in parseV2
htmlModel = structure_model(htmlPath)
File "C:\Users\keshav.bohra\PycharmProjects\git\content-comparison-advisor\minton\dashboard\file_compare\html_model_creation.py", line 342, in structure_model text_tagged = tagContent(content, Flag=True)
File "C:\Users\keshav.bohra\PycharmProjects\git\content-comparison-advisor\minton\dashboard\file_compare\html_model_creation.py", line 100, in tagContent
return H2T.handle(content)
File "c:\Users\keshav.bohra\PycharmProjects\git\content-comparison-advisor\virtual-env\lib\site-packages\html2text\__init__.py", line 142, in handle
self.feed(data)
File "c:\Users\keshav.bohra\PycharmProjects\git\content-comparison-advisor\virtual-env\lib\site-packages\html2text\__init__.py", line 139, in feed
super().feed(data)
File "C:\Users\keshav.bohra\AppData\Local\Programs\Python\Python37\lib\html\parser.py", line 111, in feed
self.goahead(0)
File "C:\Users\keshav.bohra\AppData\Local\Programs\Python\Python37\lib\html\parser.py", line 171, in goahead
k = self.parse_starttag(i)
File "C:\Users\keshav.bohra\AppData\Local\Programs\Python\Python37\lib\html\parser.py", line 345, in parse_starttag
self.handle_starttag(tag, attrs)
File "c:\Users\keshav.bohra\PycharmProjects\git\content-comparison-advisor\virtual-env\lib\site-packages\html2text\__init__.py", line 191, in handle_starttag
self.handle_tag(tag, dict(attrs), start=True)
File "c:\Users\keshav.bohra\PycharmProjects\git\content-comparison-advisor\virtual-env\lib\site-packages\html2text\__init__.py", line 598, in handle_tag
nest_count = self.google_nest_count(tag_style)
File "c:\Users\keshav.bohra\PycharmProjects\git\content-comparison-advisor\virtual-env\lib\site-packages\html2text\__init__.py", line 877, in google_nest_count
nest_count = int(style["margin-left"][:-2]) // self.google_list_indent
ValueError: invalid literal for int() with base 10: 'au'
```

- Version by `2020.1.16`
- Python version `3.7.5`