Add the "string slice" command that follows golang spec.

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extra : rebase_source : 28a8f42aa40c53818374538451906e6c5a06aef9
Merge pull request #8 from MrWerewolf/slicer

Add the "string slice" command that follows golang spec.
commands "tag", "ht", and "ht-raw".  See cmds_test.go for example.
Accept more sequences in consumeBackslashEscaped()