Move all gitignore matching to separate crate.

This PR introduces a new sub-crate, `ignore`, which primarily provides a
fast recursive directory iterator that respects ignore files like
gitignore and other configurable filtering rules based on globs or even
file types.

This results in a substantial source of complexity moved out of ripgrep's
core and into a reusable component that others can now (hopefully)
benefit from.

While much of the ignore code carried over from ripgrep's core, a
substantial portion of it was rewritten with the following goals in
mind:

1. Reuse matchers built from gitignore files across directory iteration.
2. Design the matcher data structure to be amenable for parallelizing
   directory iteration. (Indeed, writing the parallel iterator is the
   next step.)

Fixes #9, #44, #45

appveyor.yml

@@ -30,6 +30,7 @@ test_script:
   - cargo test --verbose
   - cargo test --verbose --manifest-path grep/Cargo.toml
   - cargo test --verbose --manifest-path globset/Cargo.toml
+  - cargo test --verbose --manifest-path ignore/Cargo.toml
 
 before_deploy:
   # Generate artifacts for release
@@ -59,7 +60,8 @@ deploy:
 
 branches:
   only:
-    - appveyor
-    - /\d+\.\d+\.\d+/
-  except:
     - master
+    # - appveyor
+    # - /\d+\.\d+\.\d+/
+  # except:
+    # - master