Added bootstrap4 specific templates

Author: srtab

src/templates/bootstrap4/checkbox.html

@@ -0,0 +1 @@
+{% include 'multiple_input_option.html' %}

src/templates/bootstrap4/file.html

@@ -0,0 +1,5 @@
+<label class="custom-file">
+    {% set attrs = attrs|merge({'class': 'custom-file-input'}) %}
+    {% include 'input.html' %}
+    <span class="custom-file-control"></span>
+</label>

src/templates/bootstrap4/multiple_input.html

@@ -0,0 +1,5 @@
+<div id="{{attrs.id}}" class="custom-controls-stacked">
+    {% for option in options %}
+        {% include option.template with option only %}
+    {% endfor %}
+</div>

src/templates/bootstrap4/multiple_input_option.html

@@ -0,0 +1,6 @@
+<label for="{{for|e}}" class="custom-control custom-{{ type|e }}">
+    {% set attrs = attrs|merge({'class': 'custom-control-input'}) %}
+    {% include 'input.html' %}
+    <span class="custom-control-indicator"></span>
+    <span class="custom-control-description">{{ label|e }}</span>
+</label>