Add option to respect original Markdown raw format in po2md

[dingyifei]

The numerical list turned to 1.1.1.1... in 0.3.56, which doesn't exist in 0.3.19 (which I've used in the past)

(The render is still correct, it's just the raw markdown being different)

[dingyifei]

Additionally, I find it reformating the tables and adding empty lines. ( I'm not sure it is the intended behavior.)
The output tables

The output with extra empty lines

The original file

[mondeja]

Pull requests are welcome. Two outputs in all cases provides the same HTML AST, so I'm not worried about it. This issue is the same as the closed #92

It would be useful a disabled by default option --raw-format/raw_format to respect the original Markdown formatting.

[mondeja]

From a technical perspective, I found this impossible to implement if MD4C parser doesn't include information about syntax. I've created an Implementation notes section in documentation to track all differences in syntax defined by mdpo for technical reasons and parser limitations that should be completed by who would want to implement this feature.