Added option to search for the word which is being looked up in the buffer.

Hi

I added the option to put the word which is being looked up in the Vim search register so that when one hints `n` in the documentation buffer the looked up word is searched.

Introduced the variable *g:viewdoc_copy_to_search_reg* for enabling this
feature.

BTW this is a really nice plugin. I use it all the time. :+1: