[Feature] Numbered TOC: --numbered list|outline #92

Is your feature request related to a problem? Please describe.

The TOC is always a bullet list (* [...]), and there is no way to get a numbered TOC.
The same request for the Bash version was ekalinin/github-markdown-toc#26, and it is
implemented there in ekalinin/github-markdown-toc#177.

Describe the solution you'd like

The same option as in the Bash version: --numbered <TYPE> with two types.

• list: a markdown ordered list, every marker is 1.. GitHub numbers the items
  itself, and shows nested lists as i., then a.:

  1. [Title one](#title-one)
     1. [Section](#section)
  1. [Title two](#title-two)
  

• outline: bullets with a hierarchical number at the start of the link text:

  * [1. Title one](#title-one)
     * [1.1. Section](#section)
  * [2. Title two](#title-two)
  

  The number follows the heading tree: the parent of an entry is the nearest previous
  entry with a lower level. So a skipped level does not add a part: # then ###
  gives 1. and 1.1., not 1.0.1..

Behavior in the Bash version:

• Without --numbered the output does not change.
• Only the entries kept by --depth are numbered.
• With several inputs, the numbers of each document start from 1..
• An unknown type is an error with exit code 1.
• --numbered list with --indent less than 3 is an error: a child of 1. is nested
  only when it is indented 3 to 6 spaces more than the parent, otherwise GitHub renders
  one flat list.

Describe alternatives you've considered

• Real counters in list (1., 2., 3.): GitHub uses only the first number of a
  list, and a two-digit marker (10.) needs an indent of 4 for its children.
• A number before the link (* 2.1. [Section](#section)): GitHub parses 2.1. as an
  ordered list marker inside the bullet.
• One counter per heading level instead of the tree: it gives the same number twice for
  #, ###, ## (1.1. and 1.1.).

Additional context

Differences from the Bash version to decide on:

• The default --indent here is 2, so the Bash rule above would make
  --numbered list fail without an explicit --indent 3.
• Entries here are indented relative to the top level of the document and to
  --start-depth. Entries skipped by --start-depth should not be numbered either.