Will have to fix this in a better way later in relation to the non-renderable pages.
But this commit brings the Hugo Benchmark down to "only slightly slower" than master.
```
benchmark old ns/op new ns/op delta
BenchmarkHugo-4 10074504521 10071236251 -0.03%
benchmark old allocs new allocs delta
BenchmarkHugo-4 43623091 49271859 +12.95%
benchmark old bytes new bytes delta
BenchmarkHugo-4 9468322704 9725848376 +2.72%
```
Which is something we can work with.
This is a pretty fundamental change in Hugo, but absolutely needed if we should have any hope of getting "multiple outputs" done.
This commit's goal is to say:
* Every file target path is created by `createTargetPath`, i.e. one function for all.
* That function takes every page and site parameter into account, to avoid fragile string parsing to uglify etc. later on.
* The path creation logic has full test coverage.
* All permalinks, paginator URLs etc. are then built on top of that same logic.
Fixes#1252Fixes#2110Closes#2374Fixes#1885Fixes#3102Fixes#3179Fixes#1641Fixes#1989
Changing it to PageOutput was a mistake. You may think that the increased parallelism should be a good thing.
But not so much with the increased lock contention and more complex concurrency model.
This page was a real chore chore on the POC. In the event that a new doc site won't be approved by v20, this will at least clean up the broken links I found during my audit. I would like to eventually create this as a [data file](https://github.com/spf13/hugo/pull/2508#commitcomment-18705617) since md tables get unwieldy pretty quickly.
Difference between toml.Load(string(datum)) and
toml.LoadReader(bytes.NewReader(datum)):
benchmark old ns/op new ns/op delta
BenchmarkLoad-4 82068 78489 -4.36%
benchmark old allocs new allocs delta
BenchmarkLoad-4 494 493 -0.20%
benchmark old bytes new bytes delta
BenchmarkLoad-4 17009 16913 -0.56%
Add vendoring and make targets. Require Go 1.8+ and remove mention of
GOPATH (just assume $HOME/go).
Due to time contraints, the contributing tutorial was only slightly
updated to make obvious corrections.
Fixes#3153
