Allow different highlighting for indentation related whitespace

Currently I'm using the following overrides for Ale's highlights ([link to dotfiles](https://github.com/ismay/dotfiles/tree/453ea28f1efbb9149f7756afc4ffd608ab80613b) with the settings for more details):

```vim
hi ALEError cterm=undercurl ctermul=red
hi ALEWarning cterm=undercurl ctermul=yellow
```

Now these work great, but they highlighted something that I thought would be a nice potential feature for ale. If you'll look at the screenshot below:

you can see that the highlight is also highlighting the whitespace in front of the tag. I was wondering if it'd be possible for Ale to detect indentation and allow setting a different highlight for that. Say `ALEErrorIndentation`, `ALEWarningIndentation` or something of that nature. That way I could disable the undercurl there and not be as distracted by the highlighted indentation. Which could look like, for example, this:

Or alternatively, I could also imagine having a setting that allows you to prevent Ale from highlighting indentation related whitespace at all. I'm not sure what the technical possibilities are and whether it's in scope, but for readability I do think it would be nice.