Strip ANSI codes from page based on config - fixes #79

[sumpygump]

Adds new boolean config option ansi in [a-general] to switch whether ANSI codes in page content will be rendered to terminal.

If ansi is set to false or color is set to false, any ANSI codes will be stripped from fetched content.
Fixes #79

Here is a summary of results for the various values of the two options:

config: color	config: ansi	Result
true	true	colorize headings & links; translate ANSI
true	false	colorize headings & links; strip ANSI
false	true	no color; strip ANSI
false	false	no color; strip ANSI

Notice how it is not possible to have ansi color codes true and color false at the same time. If the color config option is false, it overrides the value of ansi.

Feature to only allow ansi codes in preformatted blocks is part of another issue (#59)

[makew0rld]

Thanks, this is helpful! I have a few issues.

• What is the source for that regex? You should put it as a comment in the code
• Define the regex as a package/global variable like ansiRegex or ansiRE
• Optionally strip ANSI in the RenderANSI function as well
• Add the default value of a-general.ansi to config/config.go, like all config options

You can add //nolint:lll right before the regex variable to make linting pass, I don't mind in this case.

[sumpygump]

Thanks for the tips. I made your suggested changes.

[makew0rld]

The ansiRegex variable should be the output of regexp.MustCompile, not a string. Since the stripANSI function would just become one line, you can replace it with just using ansiRegex.Replace....

Also I'd appreciate if you pointed to the source directly:

// Source: https://github.com/acarl005/stripansi/blob/2749a054d4758fb0126d83aae590d7c9e8982d77/stripansi.go

On another level, I wonder if there's any point in stripping non-color codes only sometimes. The way things are done currently, those codes will be displayed on the screen when ANSI is enabled, but stripped when it's disabled. It might make sense to only strip color codes, since the move and other codes don't actually affect the terminal display anyway, thanks to cview.

A regex like `\x1b\[[0-9;]*m` (note it's a backtick string) should be able to just get the color codes. That's taken from here, which I linked in #79. I'm not sure why the stripansi regex is so long compared to these other ones. I've seen this longer version in other places like here, but there seems to be no complaints about the shorter ones on superuser.com.

[sumpygump]

Okay, that makes sense. I was thinking that the goal would be to strip all ansi codes, worrying that things like clear screen and move would potentially mess up the rendering of the page. I'll switch it to the simpler regex.