Add html_filename_parameterization option.

This adds the vimwiki_local `html_filename_parameterization` option (0|1)
which performs the same sanitization of filenames as does the
vimwiki_markdown gem.

For example, if your file is called "My File.md" it will be written out
as "my-file.html".

If the html_filename_parameterization options is enabled, we also _do
not delete_ non-matching html files when the VimwikiAllToHtml command is
run.

Closes #129

autoload/vimwiki/vars.vim

@@ -281,6 +281,7 @@ function! s:populate_wikilocal_options()
         \ 'template_default': {'type': type(''), 'default': 'default', 'min_length': 1},
         \ 'template_ext': {'type': type(''), 'default': '.tpl'},
         \ 'template_path': {'type': type(''), 'default': $HOME . '/vimwiki/templates/'},
+        \ 'html_filename_parameterization': {'type': type(0), 'default': 0, 'min': 0, 'max': 1},
         \ }
 
   let g:vimwiki_wikilocal_vars = []