Better (?) location for transient files

Closes #22.

README.markdown

@@ -31,9 +31,10 @@ mostly `:set` calls.)  Here's a taste:
 * `'incsearch'`: Start searching before pressing enter.
 * `'listchars'`: Makes `:set list` (visible whitespace) prettier.
 * `'scrolloff'`: Always show at least one line above/below the cursor.
-* `'directory'`: Dump swap files under `~/.cache/vim/swap` (only if that
-  directory already exists).  Also `'backupdir'` and `'undodir'`.  Try
-  `mkdir -p ~/.cache/vim/{swap,backup,undo}`.
+* `'directory'`: Dump swap files under `~/.local/share/vim/swap`,
+  `~/Library/Vim/swap`, or `~\Application Data\Vim\swap`, depending on your
+  platform.  Also `'backupdir'` and `'undodir'`.  The directories aren't
+  created automatically, so try `mkdir -p ~/Library/Vim/{swap,backup,undo}`.
 * `nmap Y y$`: Make `Y` behave like `C` and `D`.
 * `runtime! macros/matchit.vim`: Load the version of matchit.vim that ships
   with Vim.

plugin/sensible.vim

@@ -60,15 +60,16 @@ if &history < 1000
 endif
 set viminfo^=!
 
-if isdirectory(expand('~/.cache/vim'))
+let s:dir = has('win32') ? '~/Application Data/Vim' : has('mac') ? '~/Library/Vim' : '~/.local/share/vim'
+if isdirectory(expand(s:dir))
   if &directory =~# '^\.,'
-    set directory^=~/.cache/vim/swap//
+    let &directory = s:dir . '/swap//,'
   endif
   if &backupdir =~# '^\.,'
-    set backupdir^=~/.cache/vim/backup//
+    let &backupdir = s:dir . '/backup//,'
   endif
   if exists('+undodir') && &undodir =~# '^\.\%(,\|$\)'
-    set undodir^=~/.cache/vim/undo//
+    let &undodir = s:dir . '/undo//,'
   endif
 endif
 if exists('+undofile')