Do not clobber default register when initializing tree buffers and do not fix window width when re-using working buffer windows

- Fix issue https://github.com/troydm/easytree.vim/issues/7 by using the "black hole" register when clearing the tree buffer.
- When using '`EasyTreeHere`', the window width is no longer adjustable due to the '`set local winfixedwidth`' command being issue in the buffer window. This fixes it by only fixing window width if EasyTree is opened in its own split.