Enable turning off limit on maximum number of signs

Setting the variable to a known value (-1) is neater than an arbitrary
high number.

See airblade#681.

README.mkd

@@ -143,6 +143,8 @@ To keep your Vim snappy, vim-gitgutter will suppress the signs when a file has m
 let g:gitgutter_max_signs = 500  " default value
 ```
 
+You can also remove the limit by setting `g:gitgutter_max_signs = -1`.
+
 #### Hunks
 
 You can jump between hunks:

autoload/gitgutter/diff.vim

@@ -182,7 +182,7 @@ function! gitgutter#diff#handler(bufnr, diff) abort
   let modified_lines = gitgutter#diff#process_hunks(a:bufnr, gitgutter#hunk#hunks(a:bufnr))
 
   let signs_count = len(modified_lines)
-  if signs_count > g:gitgutter_max_signs
+  if g:gitgutter_max_signs != -1 && signs_count > g:gitgutter_max_signs
     call gitgutter#utility#warn_once(a:bufnr, printf(
           \ 'exceeded maximum number of signs (%d > %d, configured by g:gitgutter_max_signs).',
           \ signs_count, g:gitgutter_max_signs), 'max_signs')

doc/gitgutter.txt

@@ -419,6 +419,8 @@ signs, so to avoid slowing down the GUI the number of signs is capped.  When
 the number of changed lines exceeds this value, the plugin removes all signs
 and displays a warning message.
 
+Set to -1 to remove the limit.
+
                                                    *g:gitgutter_sign_priority*
 Default: 10