Added simple auto-completion to search prompt

Currently, label completions for all labels are supported, as well as
these keywords: from, to, after, date, before, date, limit, AND, OR, NOT
sup-config: call to_s() on user input

Highline seems to use its own string wrapper for data entered by the
user. When converting to yaml, the value is annotated with
'!str:HighLine::String', which leads to errors when starting sup or
sup-sync.

Fixes issue #28
Merge pull request #29 from ericweikl/sup-config-bug28

sup-config: call to_s() on user input
Merge pull request #27 from ericweikl/develop

Add auto-completion to search prompt
Merge branch 'develop' into rmail-to-mail-remove-iconv-ruby-2-comp