Adds keyword to js scanner to scan ECMA6 compliant js (#33)

[cameron-dunn-sublime]

Describe the change
Include a summary of the change (with required dependencies), any issues it fixes, and relevant motivation and context.

Originally implemented here: sublime-security#33. No merge conflicts (although that fork does have some moved around scanner config files)

Credit to @alexk307 for the change!

Describe testing procedures
I tested with an extremely minimal file test.js with contents:

let foo = 'bar'

Without this change no yara flavor is returned, and with this change the flavor javascript_file is returned.

Sample output
If this change modifies Strelka's output, then please include a sample of the output here.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of and tested my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[phutelmyer]

This looks good. I'll test it out and merge it in. Thanks @cameron-dunn-sublime and @alexk307 :)

[phutelmyer]

Tested. No issues. Thank you for your contribution!