Brighter red for dark bg

When used with a dark background in the GUI, the underline/undercurl for spelling errors were overwhelmed by the much brighter text. Left existing light bg red as is. Used iA Writer's brighter red for the 'light' red.

Added contrast option to the docs.

README.markdown

@@ -11,7 +11,6 @@ does have a nice color scheme, however.
 
 The _pencil_ color scheme features: 
 
-* Accurate colors
 * Subtle indicators of changes in the gutter for Signify, git-gutter, etc.
 * Use for both code and prose, though it’s definitely geared towards the
   latter
@@ -31,6 +30,19 @@ Or simply copy the color scheme file to your `~/.vim/colors` directory.
 
 ## Configuration
 
+### Contrast
+
+If you’re looking for greater contrast, set the following in your
+`.vimrc`:
+
+```
+let g:pencil_higher_contrast_ui = 0   " 0=low (def), 1=high
+```
+
+It currently only affects the blacks and grays.
+
+### Parentheses matching
+
 Those users who find the parentheses matching disconcerting can disable
 this default Vim plugin in their `.vimrc` with: