Implemented pretty-printing for the text back-end.

[drdozer]

I think this implements pretty-printing for the text back-end. It should be almost free for non-pretty-print renderings.

[lihaoyi]

@drdozer this looks good. Could we get a few unit tests to cover the various cases implemented in https://github.com/lihaoyi/scalatags/pull/127/files#diff-bba6bad898861f6e39c29d9f17b2f7daR186? That's all I'd want before merging this

[drdozer]

I'll see if I have some time in meetings today to do this.

[chadselph]

is this branch dead? I would like to have this feature too 😃

[nafg]

Me too :)