Create String and Silent funcs

[makew0rld]

This PR adds the ProgressBar.String() method that returns the rendered version of the progress bar as a string. This is useful for environments where one doesn't want to output to stdout, but use the progess bar somewhere else. Personally I'm using it in Amfora, which is a TUI application.

This PR also adds the DefaultSilent and DefaultBytesSilent funcs, which create a ProgressBar that doesn't output anywhere, and doesn't have

OptionOnCompletion(func() {
	fmt.Fprint(os.Stderr, "\n")
})

[makew0rld]

I will be fixing this PR and updating it so that it's mergeable. Tracking progress on that in this issue.

cc @MarvinJWendt

[MarvinJWendt]

Hey @makeworld-the-better-one, I am not working on this repository. I just wanted to point out, that your fork could not be merged because of the changed go.mod file.
AFAIK the only person who could merge this is @schollz :)

[makew0rld]

@schollz This is now ready and can be properly merged.

[makew0rld]

@schollz merge conflicts have been resolved. Can this be merged? Thanks.

[ecliptik]

@schollz ping if this could get reviewed and merged as it would help with some Debian packaging too. Thank you.

[schollz]

Sure, it seems good. I guess a test would be useful (though not sure how that would be written) but I'll trust that it works!

[makew0rld]

Thank you!