Add option to configure content rendering in block form

[nag5000]

yieldedContentRendering:

• 'always': render always (by default, old behaviour);
• 'lazy': render since the first opening;
• 'open': render only when modal is open, content is destroyed on closing.

Closes #40.

[esbanarango]

Thank you @NagiTs could you add this option to the readme.

[nag5000]

Hmmm, I would like to note that {{#modal.open}}...{{/modal.open}} сontextual сomponent will not be rendered with option yieldedContentRendering='lazy'/'open'. It seems that the problem can't be solved without breaking changes. Probably, it should be documented, if you have no other ideas.