* Attempting to fix AppVeyor builds

* Trying again, 2015 image this time

* Use Appveyor's Go 1.11 stack

* Restore GOPATH\bin to PATH and delete old image config

* Add gcc to path manually

* Proxy can now use QUIC for upstream connections

Add HandshakeTimeout, change h2quic syntax

* Add setup and upstream test

Test QUIC proxy with actual h2quic instance

Use different port fo QUIC test server

Add quic host to CI config

Added testdata to vendor

Revert "Added testdata to vendor"

This reverts commit 959512282deed8623168d090e5ca5e5a7933019c.

* Use local testdata

The vendor/ folder was created with the help of @FiloSottile's gvt and
vendorcheck.

Any dependencies of Caddy plugins outside this repo are not vendored.

We do not remove any unused, vendored packages because vendorcheck -u
only checks using the current build configuration; i.e. packages that
may be imported by files toggled by build tags of other systems.

CI tests have been updated to ignore the vendor/ folder. When Go 1.9 is
released, a few of the go commands should be revised to again use ./...
as it will ignore the vendor folder by default.

golint is not part of the tests since our Markdown dependency defines
an interface that is not lint-compliant (unfortunately).
See https://github.com/russross/blackfriday/issues/240