Validate json after save

[gianarb]

If filename is composer.json runs after saving command

composer validate

And if it return 0 (error) open horizontal split with command output

[sensorario]

Run a command after saving a file:
http://vimdoc.sourceforge.net/htmldoc/autocmd.html#FilterWritePost

Get current filename:
http://vimdoc.sourceforge.net/htmldoc/eval.html#expand()

Open horizontal split with command output
... I've seen :copen command, but I need a bit of investigation of it ...