0.4.3: Introduce a -tables option. (#45)

The `-tables` option makes it possible to specify a JSON file whose
properties define arguments that will be passed to the
`tables.OverrideTables()` function such that other projects can define
their own constraints on argument formatting. This is valuable to
Buck or Skylark users that define rules that need to be formatted in a
way that is different from buildifier's defaults.

Test Plan:
* Added a test: `tables/jsonParser_test.go`.
* Verified that `bazel test //...` succeeds.
* Created a JSON config for Buck and verified the output of the
following was what I expected when run from the Buck repo:

```
export BUILDIFIER_DIFF="diff -u"
find . -name BUCK | xargs -I {} ../buildifier/bazel-bin/buildifier/buildifier -buildifier_disable=label -tables=buildifier.json -d {}
```

For this test, the contents of `buildifier.json` were:

```
{
  "IsLabelArg": {
  },
  "LabelBlacklist": {
  },
  "IsSortableListArg": {
    "deps": true,
    "exported_deps": true,
    "provided_deps": true,
    "srcs": true,
    "visibility": true
  },
  "SortableBlacklist": {
    "genrule.srcs": true
  },
  "SortableWhitelist": {
  }
}
```