allow BeautifulSoup configuration kwargs to be specified

[chrispy-snps]

Fixes #223 by extending #206 to also support the specification of full BeautifulSoup kwargs values.

If a simple string or list is passed, the original behavior from #206 is used to treat it as a features specification:

md = MarkdownConverter(bs4_options="html.parser")
md = MarkdownConverter(bs4_options=["lxml", "html.parser"])

But if a dictionary is passed, then it is treated as a full kwargs specification:

md = MarkdownConverter(bs4_options={"exclude_encodings": ["iso-8859-7"]})

[AlexVonB]

Good solution! Since the option name changes, you should at least make this a minor release.

How do you handle the command line options? Could we just... ignore this option for the CLI? :D

[chrispy-snps]

@AlexVonB - ack! Yes, I need to update the command-line script. I say we just accept only a string value for features and call it good. If you're doing anything more complex than that, it seems reasonable to do it from code.