Improved structural variation support

It has been a really productive summer thanks to @Anishka0107, the Google Summer of Code student who has improved the support for structural variants in the validator and the debugulator 😃

She has added new metadata validations to ensure that INFO and FORMAT fields match the header definition, and that said header matches the VCF specification itself. These validations apply not only to short variants but also to structural variation tags, which hadn't been fully supported until now!

She also expanded the checks (added to last version) that guarantee no duplicate values in the ID and FORMAT columns in a single line, to also include the FILTER and INFO columns. The debugulator can now automatically fix these duplicates, as well as the values assigned to some INFO tags (see #78 for more details).

The last phase of GSoC was more focused on the purely technical aspects of the project: cleaning up the code, improving the documentation and slightly simplifying the grammar that detects syntax errors.

Please download the Linux binaries using the links below, and visit this page if you are interested in the full list of changes.