Support specifying the style of the generated unordered list markup

[brlin-tw]

The markdownlint pre-commit hook errored in my project after running doctoc because the style of the unordered list is inconsistent with the others:

$ pre-commit run markdownlint --files README.md
markdownlint.............................................................Failed
- hook id: markdownlint
- exit code: 1

README.md:20:1 MD004/ul-style Unordered list style [Expected: plus; Actual: dash]
README.md:21:1 MD004/ul-style Unordered list style [Expected: plus; Actual: dash]
README.md:41:1 MD004/ul-style Unordered list style [Expected: dash; Actual: asterisk]
README.md:44:1 MD004/ul-style Unordered list style [Expected: dash; Actual: asterisk]

It would be great if doctoc supports customizing the style of the unordered list so such inconsistency won't occur, specifically:

• The bullet type used in each level of the unordered list
• The indentation width
• The indentation type: space/tab

Thanks in advance!

[thompson-tomo]

Indentation width has been implemented, other 2 points are outstanding.

[thompson-tomo]

Bullet type support will be available via #353