* Ignore empty `key=value` strings in variable assignments. Now the
following lines are supported with the expected meaning:
* `a=1 , b=2`
* `a=1,,b=2`
* Improve the warning message when there is an unterminated quote.
* Add a warning if the line ends with a backslash.
Also add unit tests.
Now all that is needed to add a new unit test is to create a `t/*.pl`
file and list it in the `handwritten_tests` variable.
To run the test suite, run:
./autogen && ./configure && make check
Fixes#147
