Add working directory param

Example usage:

```
resources:
- name: <resource name>
  type: <resource type name>
  source:
    access_key_id: {{aws-access-key}}
    secret_access_key: {{aws-secret-key}}
    bucket: {{aws-bucket}}
jobs:
- name: <job name>
  plan:
  - task:
       config:
          platform: linux
          image_resource: *node
          outputs:
              - name: built-project
  - put: <resource name>
     params:
       dir: built-project
```

README.md

@@ -33,6 +33,16 @@ jobs:
 
 The `access_key_id` and `secret_access_key` are optional and if not provided the EC2 Metadata service will be queried for role based credentials.
 
+## Params
+
+Parameters for setting up the `aws s3 sync` command include:
+
+```
+- put: push-to-s3
+  params:
+    dir: subdirectory # will change the working directory to subdirectory before invoking the sync
+```
+
 ## Options
 
 The `options` parameter is synonymous with the options that `aws cli` accepts for `sync`. Please see [S3 Sync Options](http://docs.aws.amazon.com/cli/latest/reference/s3/sync.html#options) and pay special attention to the [Use of Exclude and Include Filters](http://docs.aws.amazon.com/cli/latest/reference/s3/index.html#use-of-exclude-and-include-filters).

assets/out

@@ -19,6 +19,7 @@ payload=`cat`
 bucket=$(echo "$payload" | jq -r '.source.bucket')
 path=$(echo "$payload" | jq -r '.source.path // ""')
 options=$(echo "$payload" | jq -r '.source.options // [] | join(" ")')
+dir=$(echo "$payload" | jq -r '.params.dir // "."')
 
 # export for `aws` cli
 AWS_ACCESS_KEY_ID=$(echo "$payload" | jq -r '.source.access_key_id // empty')
@@ -34,8 +35,9 @@ fi
 # Export AWS_DEFAULT_REGION if set
 [ -n "$AWS_DEFAULT_REGION" ] && export AWS_DEFAULT_REGION
 
-echo "Uploading to S3..."
-eval aws s3 sync $source "s3://$bucket/$path" $options
+cd "$source/$dir"
+echo "Uploading to S3 from '$dir'..."
+eval aws s3 sync $source/$dir "s3://$bucket/$path" $options
 echo "...done."
 
 source "$(dirname $0)/emit.sh" >&3