Add run formatting support

[AlexanderVatov]

I have implemented support for adding runs with formatting (bold, italic, underline, superscript, subscript, small caps, and shadow are supported).

[amiremohamadi]

@AleksanderVatov thanks for the PR! 😃
It seems good to me but there's a few things:

1. we prefer to name variables and method parameters snake_case
2. we prefer to only write the type of the parameter in method declaration. so it would be good to avoid writing parameter names in .hpp files
3. it would be great to create a new .cpp file in samples/ directory and write an example of how to use run-formatting

[AlexanderVatov]

Will do. Can I combine this with the issue I wrote about, with the leading and trailing spaces in runs?

As for the parameter name, I noticed that you don't include them in the header file, but here in particular I thought it might be a good idea because its purpose (as a combination of formatting flags) is otherwise unclear. I can then overload the & and | operators and use the FormattingFlags type.

[AlexanderVatov]

I overloaded the &, |, &=, and |= operators; now FormattingFlags can be used instead of int, and thus the parameter's name is no longer necessary in the header. I also added a sample.

[AlexanderVatov]

@amiremohamadi I have fixed #50 and implemented two new methods in another branch (https://github.com/AleksanderVatov/DuckX/tree/append_paragraph); please accept this PR first I'll make a new one for that branch.

[amiremohamadi]

Can I combine this with the issue I wrote about, with the leading and trailing spaces in runs?

It's better to open separate pull-requests but I'll merge this pull-request soon
Thanks a lot for your efforts :-)