Add ability to ignore help buffers

[bwells]

I'd like the ability to disable hardtime while in a help buffer. Generally I find non-optimal patterns like holding down j to scroll within buffers that are mostly just text to be just fine.

I'm happy to code it up and submit a pull request. Do you have any preference on implementation?

We could add g:hardtime_ignore_help and s:IsIgnoreHelp() just like the quickfix counterparts, or combine the implementations of s:IsIgnoreQuickfix and call it something like s:IsIgnoreSystem

[takac]

Why would you want to disable it in help? It is one type of buffer where efficient movements should be easy.