Support for SAXON_CUSTOM_OPTIONS in command line

[AirQuick]

This pull request derives from #267 . So needs to be handled after that.

---

Closes #235

This pull request mimics the saxon.custom.options property of Ant.
With it, you can set the environment variable SAXON_CUSTOM_OPTIONS before running xspec.bat or xspec.sh. For example, if you do

C:\test>set "SAXON_CUSTOM_OPTIONS=-config:"test config.xml" -t"

C:\test>bin\xspec test.xspec
...

or

$ export SAXON_CUSTOM_OPTIONS='"-config:test config.xml" -t'
$ bin/xspec.sh test.xspec
...

and then -config:... and -t are passed to Saxon and take effect when running the test in test.xspec.
(The options do not take effect when compiling the test or formatting the test result, which is the same as Ant.)

[cirulls]

This looks good to me. I reviewed it as follows:

• I ran successfully the bats tests locally on Linux and Windows and made sure that the new tests are executed ('invoking xspec.sh for XSLT with SAXON_CUSTOM_OPTIONS' and 'invoking xspec.sh for XQuery with SAXON_CUSTOM_OPTIONS').

• I made a test failing on purpose to check that errors are caught. It failed as expected.

• I inspected the code and read the discussion in Is it possible to apply Saxon configuration files?  #235.

• Since @duenckel successfully tested it manually with xspec.bat, I tested it manually with xspec.sh in bash. First, I set up the environment variable:

  export SAXON_CUSTOM_OPTIONS='"-config:test/saxon-custom-options/config.xml" -t'

  Then I ran successfully the test:

  bin/xspec.sh test/saxon-custom-options/test.xspec

  I also tested that this works with SAXON EE. To do this, in test/saxon-custom-options/config.xml I changed the edition attribute to edition="EE" and added the serialization configuration which is supported only by Saxon EE or PE:

   <serialization method="xml" indent="yes" saxon:indent-spaces="8" xmlns:saxon="http://saxon.sf.net/"/>

  The test ran successfully with SAXON EE and I also simulated an error. Since we can't run tests on the CI systems with SAXON EE at the moment, we can't add this additional test to the test suite (but I wanted to check it manually any way).

@AirQuick: As soon as you rebase the branch of this pull request on the current master (I pushed a commit to master since your last commit), I will merge this into master.

Just a question (which does not impact the merge): Saxon custom options are now supported when running XSpec for XSLT and XQuery. Would we need something similar for running XSpec for Schematron? Or is it not relevant for the Schematron support?

[AirQuick]

Would we need something similar for running XSpec for Schematron?

Since XSpec for Schematron is eventually converted to XSpec for XSLT, we don't need to add anything. Saxon custom options take effect when testing Schematron just fine. (And, for the same reason, I don't think we need a dedicated test.)

[AirQuick]

Added the variable to Wiki.

[cirulls]

Thanks for the clarification and for updating the documentation.