Add support for preinit_hook command for provider

#21

Add support for passing ssh command to be executed

NEW FEATURES:

[GH-13]: Implemented --ssh-command flag for ssh command for remote execution

EC2 provider improvements

• EC2 provider: Fallback to private IP/DNS when the public one is not available

Add missing support for extra ssh/scp params

BUG FIXES:

• [GH-11]: Add missing ssh/scp extra args support

Minor bugfixes

BUG FIXES:

• [GH-6]: AwsEc2Cache Refresh() fix
• [GH-8]: Print errors when checking for the newer version
• [GH-9]: Fix cache refresh messages

OTHER:

• [GH-7]: Enable unit-tests and test PR building with Travis

Minor fixes after public release

NEW FEATURES:

• Check for the newest version when using --verbose flag

BUG FIXES:

• [GH-1]: execute CheckForNewVersion after parsing flags
• Add warning message when no providers configured
• Fix joining paths when creating cache files
• Fix config file example

Public release

v1.0.0

Version 1.0.0 for public release