describe open-asset for javascript_include_tag
implement open-asset command
describe open-asset command when it specifies name with slash
add vendor files into fixtures
implement open-asset when it includes in vendor directory
implement open-asset command to open asset in public directory
implement open-asset command to open stylesheet
add open-asset usage into README.md
Merge pull request #17 from hmatsuda/open-assets-command

Implement open-asset command