Adding "pod cache" command

Added recursive support for header sub-directories

- Fixed file permissions on file_accessor.rb

- Simplified the regular expression logic for detecting .framework
related header files.

Fixed issue caused by the original fix for #3161, where build and
public headers were included in the Public/Headers/SpecName directory,
even if they were correctly included also in the
Public/Headers/SpecName/FrameworkName directory as the fix for #3161
was addressing. The headers defined in either source_files and
public_header_files will be ignored if they are already included by the
vendored_frameworks.

- Updated banana-lib.tar.gz to include a SubDir/SubBananalib.h in the
Bananalib.framework.

- Updated broken specs because of the new SubDir/SubBananalib.h header,
and also expanded the new vendor spec added for #3161 to include a
validation for the pretense of SubDir/SubBananalib.h.

First draft, Still WIP (TODO = Add specs + add `pod cache clean` command)

[Create] Allow passing additional args to the configure script

[Validator] Explain why linting failed

Add a pre-install hook

Make updating specs repos its own UI section

[Cache] Remove cache when CocoaPods version changes

Fixes #3161