Add !default to sass config

I suggest adding !default to the config.

$bem-config: (
  "prefix": "-",
  "element": "__",
  "modifier": "--"
);

It would look like this:

$bem-config: (
  "prefix": "-",
  "element": "__",
  "modifier": "--"
) !default;

I'm using bootstrap and my configuration file is at the beginning of imports. "Bootstrap" has all its variables with "!default", so it is easy to overwrite them. Could "bembem" adopt the same criteria?

Excuse my english.