Fail CircleCI builds when files have syntax errors

[nareddyt]

Fail a CircleCI build when a file has a syntax error. We should check all of the following file types:

• .js
• .json
• .html
• .css

The most important files to check would be in the /src/ directory. These are the code for the Chrome Extension, and we won't know that it's failing if we were to deploy it to users.

[LucaBozzetto]

If I'm not wrong this could be fixed adding a -run command and check that all files provided have the required extension.
If I'm right i can easily modify the config.yml file in order to check the files extension.

[nareddyt]

Hey! Yes, we could check the file extension but that won't tell us if there's a syntax error in the actual file.

For example, json file could be formatted because a contributor accidentally deleted a comma. The circleci build will still pass, but the extension would fail at run-time due to the incorrect JSON.

Based on the file extension, we need some way of checking if the syntax is correct for that file. Does that make sense?