Added chunkexports extension

[gatufo]

An extension to break item exports into chunks.

Settings:

• CHUNKED_FEED_URI: The feed uri to use for exporting (Overrides FEED_URI setting).
• CHUNKED_FEED_FORMAT: The feed format to use for exporting (Overrides FEED_FORMAT setting).
• CHUNKED_FEED_ITEMS_PER_CHUNK: Number of items included in each chunk.
• CHUNKED_FEED_TIMESTAMP_FORMAT: The format to be used for the timestamp uri parameter.

Example:

    CHUNKED_FEED_URI = 'export_%(chunk_number)02d.json'
    CHUNKED_FEED_FORMAT = 'json'
    CHUNKED_FEED_ITEMS_PER_CHUNK = 100

For 250 items will generate the following files:

• export_01.json (100 items)
• export_02.json (100 items)
• export_03.json (50 items)

Available uri format values:

• chunk_number: The active chunk counter. (Starts in 1).
• scrapy_job: The Scrapy job (if available).
• scrapy_project_id: The Scrapy job id (if available).
• timestamp: Current timestamp in UTC (formatted with CHUNKED_FEED_TIMESTAMP_FORMAT setting).

[redapple]

Hey @gatufo , what do you think of scrapy/scrapy#1545 ?