Use app's WebURL to decide which URL to open/show
Use IsANSI() to detect ANSI-compatible output terminal
make IsTerminal() work on Windows
prompt for confirmation on destroy

Accepts confirmation via pipe to stdin, or prompts the user to confirm.

Fixes #90.
prompt for confirmation on addon-remove
Respect $HEROKU_HOST when determining Git Host.

Fixes #100
Move hk credential and client management to a package: hkclient

When writing hk plugins or a lot of small client software, figuring
out how to get credentials is a burden.

To help with this, move all the credential and HTTP endpoint discovery
and configuration logic into a package that can be imported elsewhere,
with the explicit goal of with having closely-coupled parity with
'hk'.