Improve container

[matplane]

I suggest this improvement on the .container mixin. I think it's a nice way to use containers with multiple grid system.

Suggested new mixins with @gutter param.

My pull request is ready if you are ok with that.

.container-fixed(@gutter: @grid-gutter-width) {
  margin-right: auto;
  margin-left: auto;
  padding-left:  (@gutter / 2);
  padding-right: (@gutter / 2);
  &:extend(.clearfix all);
}

.container(@gutter: @grid-gutter-width) {
  .container-fixed(@gutter);

  @media (min-width: @screen-sm-min) {
    width: @container-sm;
  }
  @media (min-width: @screen-md-min) {
    width: @container-md;
  }
  @media (min-width: @screen-lg-min) {
    width: @container-lg;
  }
}

[mdo]

Nice catch—this completes the custom grid mixins. Didn't realize we were just missing this one part.

Worth noting though that your calling of the mixing with the variable isn't necessary for the base styles. Given that @grid-gutter-width is the default value, it'll be there unless you customize it.