option to configure tab length

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the tab length seems to be 2, which causes the following list:

- foo
    - bar
    - baz

to be formatted like so:

- foo
  - bar
  - baz

this causes the nested list to be rendered incorrectly on mkdocs (see mkdocs/mkdocs#545):

• foo
• bar
• baz

this is apparently intentional behavior because the spec states that tab length should be 4, and that every implementation of markdown that allows 2 is deviating from the spec. see Python-Markdown/markdown#3 (comment)

i don't really have an opinion on which is correct, but an option to configure it would be nice