Vim: Run without --dangerous by default

The plugin now supports running commands with an optional "!":

- "MaxdownCompile!" will clean any previously compiled artifacts before
  building the command-line tool. "MaxdownCompile" does not perform
  cleaning (this is the same as the previous behavior).
- "MaxdownConvert!" converts the current buffer with "--dangerous" mode
  enabled. The default "MaxdownConvert" no longer uses "--dangerous".
- "MaxdownPreview!" uses "--dangerous" in the same way. The default
  "MaxdownPreview" no longer does.

See the Markdown Rust crate for details about what dangerous mode means:
https://github.com/wooorm/markdown-rs

Rework I/O and read from stdin by default

By default, we now expect Markdown input to be provided on stdin if no
file path is provided, instead of interpreting "-" as stdin.

Note: This means the command no longer outputs the help content if no
arguments are provided. Instead, it will wait for input from stdin.
Users need to pass `-h` or `--help` to receive help.

Also: We now use the `std::io::Write` trait to harmonize writing command
output to file destinations and stdout.

Also: Added an extra test-case for non-writable destination files.