Support `xs file.js` instead of `xs -s file.js`

[mathiasbynens]

All other JS engines provided by jsvu support running scripts by simply passing a file name with no flags:

v8 file.js
chakra file.js
spidermonkey file.js
jsc file.js

However, xs needs the -s flag:

xs -s file.js

...because "without -e, -m or -s, strings are paths to test262 cases or directories".

Since the common case for external users seems to be running scripts, and not tests, can this be reconsidered? Specifically, I'd suggest the following changes:

• xs file.js defaults to an implicit -s, and runs file.js as a script
• xs file.mjs defaults to an implicit -m, and runs file.mjs as a module (matching V8, JSC, and Node.js)
• a new flag is added to cover the Test262 cases/directories use case

WDYT?

[acutmore]

This seems to be working now ./xs now prints that the cli args are only considered paths to test262 tests if ../harness exists.