Check syntax and semantics quickly and automatically

[rhysd]

Crystal compiler can check syntax and semantics of the code using --no-codegen. I set the check on BufWritePost event here using vim-quickrun but it is incomplete because of the entry point of the program. (Pointed out in #15)

I think it is the best to generate temporary file which requires both current file and spec (if exists). vim-crystal can support it.

• Add CrystalLint {file} command to use this feature explicitly.
• Add auto linting at BufWritePost. It can be controlled by g:crystal_auto_lint.

[rhysd]

At least this repository already has a config for syntastic. Only :CrystalLint may need.