Add --quiet flag

Hi,

Implements #1764.

I noticed that running `rake test` returns a failure:

```
[ 93/311] TestGeneratedSite#test: generated sites should ensure post count is as expected.  = 0.23 s
  1) Failure:
test: generated sites should ensure post count is as expected. (TestGeneratedSite) [/users/*****/desktop/dev/oss/jekyll/test/test_generated_site.rb:17]:
<37> expected but was
<38>.
```

Apparently the test is failing because a2fd8ba7c32aa9263754f47a1159a9c24c6bff14 added a file (named `2013-12-20-properties.text`) in `source/_posts` directory without adjusting the number to 38. However, I thought I would leave this up to you guys to change.