Add ability to control tab-size

[gabrielflorit]

For some inexplicable reason browsers render pre tab sizes with 8 spaces (8!). I'd personally like to set this to 2, e.g.

pre {
  -moz-tab-size: 2;
  tab-size: 2;
}

but I'm sure others will prefer 4. What do you think is the best way to handle this? User config in indoc.json?

[zlsa]

With the way the code is laid out, it's inherently impossible to copy more than a few lines of code. I think the cleanest solution is to convert tabs to spaces during processing; and yeah, a line in indoc.json seems to make the most sense.