Allow volofile.js for those who want to specify file format

[zanona]

I think it's nice to have tasks defined on volofile, however it would be nice if, in addition to that, volo also recognised volofile.js for users adhering to file naming conventions declaring extensions, since it is a js file.

This would also help syntax highlighting on github repositories and make easier for people to read and understand the code. gulpfile.js is a good example…