Fixes #140 and #142

* Added PromptForHiddenInput_confirm() to allow for confirmation logic
* Edited updateCheckPassphrase() in before.go to check if the store is
  being created, and if so, confirm the passphrase
* Added bool argument to AskPassphrase to allow for confirmation
  (context.go)
* go fmt, go vet
Fixed error in passphrase confirmation

* Changed "passphrase" to "create" in before.go, line 47 - now
  checks if the user is creating a store, and if so runs
  PromptForHiddenInputConfirm()
* Changed variable names in PromptForHiddenInputConfirm
* Added constants in cli_helpers.go
Merge pull request #147 from jd1123/140-blank-passphrase

Fixes #140 and #142