Added ask-passphrase flag to global options

* context.go - added global variable gAskPassphrase and added
  PromptForPassword() to get passphrase
* cmd/trousseau/before.go - updateCheckPassphrase() to update global
  variable
* cmd/trousseau/main.go - added Blooflag struct to give comments in help
Changed ask-passphrase logic

* gPassphrase in context.go
* helpers in context.go
* calling AskPassphrase from before.go sets gPassphrase
Merge pull request #127 from jd1123/feature/askpassphrase

#123 & #124 Feature/askpassphrase