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This pull request contains four changes:
* added the missing `UNION` keyword
* added an ELPA conform header including license (I hope you agree with that)
* `sparql-default-base-url` is now a `defcustom`
* changed the format behaviour
    * the hardcoded csv format is now replaced by a variable `sparql-default-format` which is "csv"
    * the variable `sparql-prompt-format` indicates whether the user should be prompted for the format on each query evaluation. default is `nil`
        * if this is `t` the user gets prompted and each selection of a format is the default-value of the next prompt