Update --volume to support arbitrary target locations

[jromero]

Description

To enable further local experimentation and development for pack users of buildpacks, builders or lifecycles we would like to loosen the constraints on where volumes may be mounted with the possibility for those mounts to be writable.

Additionally, there has been feedback that the current implementation and it's automatic appending of /platform to the target location is surprising.

Proposed solution

This issue is a request to implement proposed solution as detail in buildpacks/rfcs#85.

Summary of changes:

1. Allow --volume to support any target location
2. Support a third colon mode parameter defaulting to ro. (eg. --volume=/tmp/dir:/some-dir:rw)
3. WARN when a spec'd directory (aside from /platform) is targeted.

Additional context

• Blocked by Read/Write Platform Volume Mount rfcs#85
• This feature should be documented as part of standard pack CLI commands documentation.