[FEATURE] ViewHelpers to set/get TSFE-registers

Allows accessing values previously set in TypoScript
via LOAD_REGISTER, or to set values to access elsewhere
from the registers.
Merge pull request #639 from neufeind/feature/register-VH

[FEATURE] ViewHelpers to set/get TSFE-registers
[TER] 2.0.0
Merge branch 'master' into development