New option: g:signify_disable_by_default

You can choose to disable Sy by default now. You can enable it anytime
via :SignifyToggle (or the default mapping: <leader>gt).

Closes #34.

doc/signify.txt

@@ -110,6 +110,12 @@ NOTE: If you don't change this list and Sy is activated, updating signs for a
 non-VCS file can lead to significant delay since all supported VCS will be
 tested for.
 
+============-
+
+    let g:signify_disable_by_default = 0
+
+Don't run Sy by default. You can toggle it anytime via :SignifyToggle.
+
 ============-
 
     let g:signify_mapping_next_hunk = '<leader>gj'

plugin/signify.vim

@@ -159,6 +159,10 @@ function! s:start(path) abort
     if empty(diff)
       return
     endif
+    if get(g:, 'signify_disable_by_default')
+      let s:sy[a:path] = { 'active': 0, 'type': type, 'hunks': [], 'id_top': s:id_top }
+      return
+    endif
     let s:sy[a:path] = { 'active': 1, 'type': type, 'hunks': [], 'id_top': s:id_top }
   " Inactive buffer.. bail out.
   elseif !s:sy[a:path].active