Cache files locally for parsing

Files are now cached locally in a temporary file, and the parsing
happens on that file. This allows displaying tags for remote (Netrw)
files, and it will also make access to files on remote filesystems like
NFS and sshfs faster.

Additionally, tags are now always updated when the files are saved,
deprecating the tagbar_updateonsave_maxlines option. Otherwise checking
for changes in remote files would not have been possible.

plugin/tagbar.vim

@@ -91,10 +91,6 @@ if !exists('g:tagbar_autoshowtag')
     let g:tagbar_autoshowtag = 0
 endif
 
-if !exists('g:tagbar_updateonsave_maxlines')
-    let g:tagbar_updateonsave_maxlines = 5000
-endif
-
 if !exists('g:tagbar_systemenc')
     let g:tagbar_systemenc = &encoding
 endif