internal: Nudge to ugprade to v2

If you are reading this, you have three choices:

- Upgrade to v2. Seriously, do this. It's so much better!
- Disable the nudge with the environment variable PANICPARSE_V1_NO_WARN=1
- Stick to v1.5.0.
I

Fix go get path in README.md and panic help

Using:
  go get github.com/maruel/panicparse/cmd/pp

will retrieve 1.5.0. But now that 2.0.0 is in the sumdb, I can't remove
it anymore, hence a new 2.0.1 release that fix the path in the README
and panic's help.

README.md: add link to v2.0.0 blog post

stack: fix race ToHTML address format to hex

A pointer in decimal isn't particularly useful.

stack: speed up pass through by 1.9x

Use lower level bufio functionality, work in []byte instead of string,
saving a ton of memory copy and making the pass through much faster.

Interestingly parsing became a tad slower with more memory allocation.
It is unclear to me where they occurs.

stack: check go module after GOPATH

This is an hard call. Normally localgomod should never be inside GOPATH.

Minor forward looking v2 fixes.

stack: Lower IsPtr() floor from 8MiB to to 4MiB

That's frequent to have pointer lower in the address space. This was not
well tested but will be well tested in v2. This will modify stack
bucketing, generally helping in AnyPointer mode.

README: update for 1.4.0, add screencast

stack: add support for Go 1.13beta1

It now prints 'pc' in addition to 'fp' and 'sp'.

Include unit test.

panic: make it work on go1.7.6

Replace sort.Slice() with strings.FieldsFunc().

sort.Slice() was introduced in go1.8.

I need panic to work on go1.7 to diagnose an issue with
'additional frames elided' not being printed anymore.