Add safer path operators and improve documentation

[satabin]

The documentation now does not bring the unsafe raw operators first, and explains better what are the existing operators and what to be careful about.

This also adds a new through operator, that allows for streaming handling of path matches.

This PR is a follow-up on the discussion from #666 (cc @mrdziuban)

[ybasket]

Changes look good.

I'm mildly unhappy with the naming of through as fs2 already uses that name for something similar, yet not equal and you get through(filter.through(...)). But I have no convincing alternative suggestion, toPipe and funnel both aren't really better. So if you have another good name, let's change it, otherwise merge as-is.

[satabin]

Changes look good.

I'm mildly unhappy with the naming of through as fs2 already uses that name for something similar, yet not equal and you get through(filter.through(...)). But I have no convincing alternative suggestion, toPipe and funnel both aren't really better. So if you have another good name, let's change it, otherwise merge as-is.

I kind of chose through on purpose, as it sends things to a Pipe, and this is how fs2 calls the action. I will merge and see if I can find something else before releasing.