Disable terminal italics by default, opt-in w/ var

Yes, terminal emulation is a ghetto.  Why should that surprise you?

README.markdown

@@ -64,6 +64,16 @@ If you’re looking for neutral heading colors, set the following in your
 let g:pencil_neutral_headings = 1   " 0=blue (def), 1=normal
 ```
 
+### Terminal Italics
+
+If you're using Vim in the terminal and your terminal supports italics, you 
+can opt-in to italic support for some syntax highlighting, such as comments,
+by setting the following in your `.vimrc`:
+
+```
+let g:pencil_terminal_italics = 1
+```
+
 ### Airline Theme
 
 ![airline-example](http://i.imgur.com/V39pwZq.png)

colors/pencil.vim

@@ -45,6 +45,11 @@ if ! exists("g:pencil_neutral_headings")
   let g:pencil_neutral_headings = 0
 endif
 
+" not all terminals support italics properly.  If yours does, opt-in.
+if ! exists("g:pencil_terminal_italics")
+  let g:pencil_terminal_italics = 0
+endif
+
 " Colors
 let s:black           = { "gui": "#212121", "cterm": "0"   }
 let s:medium_gray     = { "gui": "#767676", "cterm": "243" }
@@ -122,6 +127,10 @@ endif
 
 " shamelessly stolen from hemisu: https://github.com/noahfrederick/vim-hemisu/
 function! s:h(group, style)
+  " Not all terminals support italics properly. If yours does, opt-in.
+  if g:pencil_terminal_italics == 0 && has_key(a:style, "cterm") && a:style["cterm"] == "italic"
+    unlet a:style.cterm
+  endif
   execute "highlight" a:group
     \ "guifg="   (has_key(a:style, "fg")    ? a:style.fg.gui   : "NONE")
     \ "guibg="   (has_key(a:style, "bg")    ? a:style.bg.gui   : "NONE")