Breaking change: change site_title template tag

[stuartmaxwell]

See #160 (comment) for related issue.

site_title should return HTML, e.g. {% site_title %} would return:

<div>My blog's title!</div>

or {% site_title tag="h2" class="title" %} would return:

<h2 class="title">My blog's title!</h2>

get_site_title should be used to return the string.