Add option to display spaces about ansi-to-html

[rburns]

@the-duck having a toggle to preserve spaces seems reasonable. maybe outputting it as an html entity (` ') is a better option than a span element.

It's related to the existing configuration option for preserving newlines. (https://github.com/rburns/ansi-to-html/blob/master/README.md#options) It would be tempting to preserve spaces and newlines in one config key. Though, the newlines key behaviour would need to be unchanged for compatibility.

Possibly the existing newlines key and an additional spaces key. or alternatively the existing newlines key and an additional whitespace key that does both. Not certain which would be most useful.

[brettz9]

Or how about newlines, spaces, and a setting whitespace that does both?

Just in case one wishes XML support,   would be a little safer than   which may only be supported in HTML (HTML supports either).

Am eager to have an option to auto-generate CLI docs (using command-line-usage), and preserving spacing is critical for this. But your tool otherwise looks to be doing the job quite nicely!

[rburns]

Either of those solutions are likely fine. (three config keys, or two). I struggle to imagine a scenario where you would want spaces preserved, but not newlines, or the opposite.

I guess a whitespace key would have to take into account not only spaces and newlines, but also other whitespace characters like tabs.

Makes me think newlines and whitespace would be enough. For example, adding a tabs key in addition to newlines and spaces wouldn't make a lot of sense.

[brettz9]

In my PR #57 , I've just added a tabs option as well, as I do think it's utility as a separate preference can be in using it to indicate the number of non-breaking spaces that each tab should be converted into.

[brettz9]

Though of course whatever API you prefer--just demoing the idea...