Make always-showing-the-sign-column opt-in.

This is consistent with Vim's default behaviour.

README.mkd

@@ -71,6 +71,7 @@ See the customisation section below for how to change the defaults.
 You can customise:
 
 * The sign column's colours
+* Whether or not the sign column is shown when there aren't any signs (defaults to no)
 * The signs' colours
 * Line highlights
 * Whether or not vim-gitgutter is on initially (defaults to on)
@@ -93,6 +94,8 @@ To change your sign column's appearance, update your colorscheme or `~/.vimrc` l
 * For a specific appearance on terminal Vim: `highlight SignColumn ctermbg=whatever`
 * For a specific appearance on gVim/MacVim: `highlight SignColumn guibg=whatever`
 
+By default the sign column will appear when there are signs to show and disappear when there aren't.  If you would always like the sign column to be there, add `let g:gitgutter_sign_column_always = 1` to your `~/.vimrc`.
+
 
 #### Signs' colours
 
@@ -134,6 +137,10 @@ Add `let g:gitgutter_highlight_lines = 1` to your `~/.vimrc`.
 
 Your colorscheme is configuring the `SignColumn` highlight group weirdly.  Please see the section above on customising the sign column.
 
+> Why does the window flicker when the signs are redrawn?
+
+This happens on certain combinations of OS and Vim.  You can prevent the flicker by adding `let g:gitgutter_sign_column_always = 1` to your `~/.vimrc`.
+
 > Why is no sign shown if I delete the first line(s) in a file?
 
 vim-gitgutter shows removed lines with a sign on the line above.  In this case there isn't a line above so vim-gitgutter can't show the sign.

plugin/gitgutter.vim

@@ -14,6 +14,10 @@ if !exists('g:gitgutter_highlight_lines')
 endif
 let s:highlight_lines = g:gitgutter_highlight_lines
 
+if !exists('g:gitgutter_sign_column_always')
+  let g:gitgutter_sign_column_always = 0
+endif
+
 function! s:init()
   if !exists('g:gitgutter_initialised')
     call s:define_sign_column_highlight()
@@ -368,7 +372,9 @@ function! GitGutter()
     let s:hunks = s:parse_diff(diff)
     let modified_lines = s:process_hunks(s:hunks)
     let file_name = s:current_file()
-    call s:add_dummy_sign()
+    if g:gitgutter_sign_column_always
+      call s:add_dummy_sign()
+    endif
     call s:clear_signs(file_name)
     call s:find_other_signs(file_name)
     call s:show_signs(file_name, modified_lines)