Improve if statements

[sourrust]

As of commit 7f62a10, karver only evaluates logic of the if statement as a check for a variable's existence.

Example:

{% if title %}
  <title>{{ title }}</title>
{% else %}
  <title>No Title</title>
{% endif %}

It would be nice to implement some of the suggested features list in said comment.

That list being:

• basic logically comparisons (is (==), isnt (/=), >, <, >=, <=)
• elseif expressions
• format the inside of the if statement better (ie. indention)