Switch from .rgignore to .ignore.

But don't actually remove support for .rgignore until the next semver
bump.

Note that this puts us in line with the silver searcher:
https://github.com/ggreer/the_silver_searcher/pull/974

Fixes #40

src/args.rs

@@ -137,7 +137,7 @@ Less common options:
         Never use memory maps, even when they might be faster.
 
     --no-ignore
-        Don't respect ignore files (.gitignore, .rgignore, etc.)
+        Don't respect ignore files (.gitignore, .ignore, etc.)
         This implies --no-ignore-parent.
 
     --no-ignore-parent