option to set maximum help text width

I think there is an issue in the way the text width is calculated for rhelp calls.  For example:

:echo g:vimrplugin_help_w
90
:echo g:rplugin_htw
98

I think the second number should be smaller than the first... I didn't want to adjust your algorithm in case you had a motivation for this, but regardless, I would like to be able set the maximum help text width independently of the buffer width so this pull request introduces a new variable to set the maximum text width: vimrplugin_htw