create dirs for --output

Hello,

as I was using the --output option, it did not create missing directories in the given path. This behavior is expected by most of the users, til they are used to mocha reporters or nyc reporters.

Command invoked:
``` sh
npm audit --json | npm-audit-html --output ./test-reports/npm-audit/report.html
``` 
the folder npm-audit does not exist and should be generated by the module