Add pre post install handler

[tcamin]

As highlighted in issue #60 one often needs to customise the way the single pods are compiled.

This PR adds a pre post install handler that allows to customize the Pods.xcodeproj file before compilation actually starts.

A sample Podfile looks like the following:

use_frameworks!

plugin 'cocoapods-rome', :pre_post_install => Proc.new { |installer|
    # do stuff with installer, for example change SWIFT_VERSION
    installer.pods_project.targets.each do |target|
        target.build_configurations.each do |config|
            config.build_settings['SWIFT_VERSION'] = '4.0'
        end
    end

    installer.pods_project.save
}

target 'DummyTarget' do
    pod 'MyPod'
end

[jasonm23]

:+1: thanks

[orta]

This makes sense to me, can you add some docs in the README, and this is good to go 👍

[tcamin]

Sure thing, before that what about renaming pre_post_install to pre_compile (or something like that)?

“Pre compile hook” may be easier to understand, what do you think?

[orta]

I agree 👍

[jasonm23]

Sounds good to me

[tcamin]

PR updated

[orta]

Ace- sorry, forgot about this PR 🥇

But it happened 👍

thanks!