Settings customization with defaults from the Sublime jsformat

Atom has a rather limited mechanism for specifying how settings should be
presented so limited fields like brace_style have no validation at the moment.
Merge pull request #7 from plemarquand/master

Settings customization with defaults from the Sublime jsformat
Prepare 0.1.0 release
Use file's grammar to detect if it can be formatted.

Because the file extension may not always accurately describe a file's contents,
use the file's grammar. The user can now manually set the grammar of a file that
doesn't end with .json or .js and it can be formatted.

Also using the grammar to detect validity covers other javascript file
extensions such as .es6 and .jsx.
Adding support for formatting the current selection(s)