Allow breakpoint overrides

[malberts]

Fixes: #221

Tests

Without LocalSettings.php override

With cmln override

$egChameleonExternalStyleVariables = [
    'cmln-grid-breakpoints' => '(xs: 0, sm: 576px, md: 768px, lg: 992px, cmln: 1234px, xl: 1900px)'
];

Without cmln override:

$egChameleonExternalStyleVariables = [
    'cmln-navbar-horizontal-collapse-point' => 'lg',
    'cmln-grid-breakpoints' => '(xs: 0, sm: 500px, md: 800px, lg: 1000px, xl: 1900px)'
];

With everything overridden:

$egChameleonExternalStyleVariables = [
    'cmln-navbar-horizontal-collapse-point' => 'medium',
    'cmln-grid-breakpoints' => '(nothing: 0, small: 123px, medium: 456px, large: 789px, extra-large: 2000px)'
];

(This is not necessarily sufficient as there are other Bootstrap variables that might need to be adjusted to accommodate such an extreme change.)

[malberts]

Two approaches that were not possible:

• cannot simply declare $grid-breakpoints as !default. Bootstrap itself already declares it like that and in the current order BS takes precedence (= missing cmln breakpoint)
• map-merge (like in other cases) means that something will be added whereas you actually want to replace/remove existing breakpoints