Add syntastic support using spirv-val

[kbenzie]

Add SPIR-V validation support using the spirv-val tool in SPIRV-Tools to by hooking into syntastic.

Since syntastic is currently undergoing refactoring to support vim8 async jobs (vim-syntastic/syntastic#699) which requires breaking the current checker API hooks, support for validation will be delayed until this work is complete.

[kbenzie]

ale is currently accepting PR's and also supports asynchronous features in vim 8 and neovim. This seems like a more immediately beneficial target.