local mode; support using git-remote; support graphviz

new-feature: local mode; support using git-remote

• changes of CLI:

  • --asset-dir is removed.

  • --output-dir: the root git dir to push to remote, which contains
    converted assets. A git repo will be initialized in this dir.

  • --asset-output-dir: the dir to store assets. Most cases you do
    not need to set this path.

• changes of github-action:

  • md_branch is removed. Now it always pushes to the branch it
    converts md from.

• new-feature: support specifying a remote as url of the repo that
  stores converted assets.

• new-feature: if --repo is not specified, it convert md locally,
  i.e., assets are linked by relative path, and nothing is pushed to
  remote.

• test: when CI, use repo md2test to test pushing.

• refactor: extract common util func from test, such as comparing md,
  comparing images.

• refactor: use feature-list to describe platform specific convertion.

• refactor: use k3git to parse git url.

doc: add trouble shooting

change: asset does not include platform as part of path

new-featrue: convert graphviz(dot) codes to image