Support --no-output option

[yhatt]

I've refactored processing converted file, and Marp CLI allows --no-output option now. The converted result won't be outputted to anywhere.

It is useful to preview Markdown from stdin instantly.

# Previous way
echo '# Hello!' | marp -p -o - > /dev/null

# Improved way
echo '# Hello!' | marp -p --no-output

In addition, Marp CLI has improved message when processed Markdown in server mode. Previously, the outputted log has looked like saved the converted HTML to file. But actually, it is not saved.

# Previous (A file won't saved actually)
[  INFO ] [Server mode] Start server listened at http://localhost:8080/ ...
[  INFO ] ..\..\..\Documents\marp-cli\test.md => ..\..\..\Documents\marp-cli\test.html

# Improved
[  INFO ] [Server mode] Start server listened at http://localhost:8080/ ...
[  INFO ] ..\..\..\Documents\marp-cli\test.md processed.

These improvements were realized by refactoring to add FileType.Null support.