Add CLI option to specify config file

[olssonks]

Closes #432

• CLI now has option --toml_file to specify a path to a .mdformat.toml file
  • file name must be .mdformat.toml
• Partial path supplied to CLI will resolve with a couple different options
  • Defaults to current working directory
    • Ex. --toml_file=.mdformat.toml -> /path/to/cwd/.mdformat.toml
    • Ex. --toml_file=cool_directory/.mdformat.toml -> /path/to/cwd/cool_directory/.mdformat.toml
  • Using ~ at beginning of argument will using the user's home directory
    • Ex. --toml_file=~/.mdformat.toml -> /path/to/user/home/.mdformat.toml
    • Ex. --toml_file=~/cool_directory/.mdformat.toml -> /path/to/user/homt/cool_directory/.mdformat.toml