[whatwg/encoding] gb18030 encoding/decoding support (#57)

Results for a series of tests for gb18030 encoding/decoding can be found at 
https://www.w3.org/International/tests/repo/results/encoding-dbl-byte.en#gb18030

The tests can be run from that page (select the link in the left-most column) or get the tests from the WPT repo.  There is a PR at
https://github.com/w3c/web-platform-tests/pull/3195

The test check whether:
1. the browser produces the expected byte sequences for all characters in the gb18030 index after 0x9F when encoding bytes for a URL produced by a form, using the encoder steps in the specification.
2. the browser produces the expected byte sequences for miscellaneous characters not in the gb18030 index when encoding bytes for a URL produced by a form, using the encoder steps in the specification. (tests for several ranges)
3. same two types of test when writing characters to an href value
4. the browser decodes all characters as expected from a file generated by encoding all pointers in the gb18030 index per the encoder steps in the specification.
5. the browser decodes all characters as expected from a file generated by encoding miscellaneous characters not in the gb18030 index per the encoder steps in the specification. (tests for several ranges)
6. when decoding gb18030 text, the browser uses replacement characters as described by the algorithm in the Encoding spec.

The following summarises the current situation according to my testing, for major desktop browsers.  (I will be adding nightly results and perhaps other browsers in time.)  The table lists the number of characters that were NOT successfully converted by the test.

![screen shot 2016-06-20 at 15 38 10](https://cloud.githubusercontent.com/assets/4839211/16198223/0b3b87fc-36fd-11e6-9a75-6bb08f633884.png)

Notes:
* all href tests fail for Edge because characters are not converted to percent-escapes

Can we please investigate the failures to ascertain whether:
1. the browser needs to be changed
2. the spec needs to be changed
3. the test is at fault

(I have tests ready to commit for the other encodings already, and will commit those soon and raise similar issues to this one.)


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