Allow configuration of floating window appearance

Closes #807.

doc/gitgutter.txt

@@ -348,6 +348,7 @@ Hunk jumping:~
 Hunk previews:~
 
     |g:gitgutter_preview_win_floating|
+    |g:gitgutter_floating_window_options|
     |g:gitgutter_close_preview_on_escape|
 
 Terminal:~
@@ -508,6 +509,29 @@ Whether to use floating/popup windows for hunk previews.  Note that if you use
 popup windows on Vim you will not be able to stage partial hunks via the
 preview window.
 
+                                          *g:gitgutter_floating_window_options*
+Default:
+>
+    " Vim
+    {
+        \ 'line': 'cursor+1',
+        \ 'col': 'cursor',
+        \ 'moved': 'any'
+    }
+
+    " Neovim
+    {
+        \ 'relative': 'cursor',
+        \ 'row': 1,
+        \ 'col': 0,
+        \ 'width': 42,
+        \ 'height': &previewheight,
+        \ 'style': 'minimal'
+    }
+<
+This dictionary is passed directly to |popup_create()| (Vim) or
+|nvim_open_win()| (Neovim).
+
                                           *g:gitgutter_close_preview_on_escape*
 Default: 0