handle global Flags

[root360-AndreasUlm]

While trying to solve #12 with PR #15 several options were discussed.
To move the discussion out of the PR and summarize the available options I create this issue.

The short summary of the problem #12 pointed out is that the current CLI lib does not inherit global flags into several subcommands (see urfave/cli#585)

The currently available options are:

• switching to Cobra a different CLI lib (suggested by @noerw )
• creating global flags and use append() to merge subcommand flags with global flags (suggested by @techknowlogick )

To compare both options I created a branch in my fork for each of them:

• Cobra: [WIP] 12 switch to cobra #27
• current cli: added global appendable Flags (#12) #26

[root360-AndreasUlm]

After implementing #26 I found this solution to be easier and the workaround seems to work quite well.
The switch to Cobra will take more time.

[lunny]

@root360-AndreasUlm I think #26 is easier for review.

[root360-AndreasUlm]

@lunny then let's go with #26.
I'll close #15 and #27.