Parameterless custom directive line processors

Fix for #728.

This updates allows for directive line processors that do not require an
argument (#cls for example).
Updated exception string per review of #732
Merge pull request #732 from ryanrousseau/728

Parameterless custom directive line processors
GetDirectiveArgument should trim all whitespace
Merge pull request #733 from ryanrousseau/728

DirectiveLineProcessor.GetDirectiveArgument should trim all whitespace