allow user to set split size

I added the ability to configure the split size for the results window through
an option in .vimrc.

The option `g:RspecSplitSize` defaults to 15, which is the split size used
in the code this commit replaces. Users who don't set `g:RspecSplitSize` in
their vimrc won't see any change in behavior, while users who prefer a
larger results pane can set the split size higher.