Provide substring functions for template

[bjendres]

We want the following functions to be available in (our) smarty interpreter:

• mg_startswith($string, $prefix)
• mg_endswith($string, $suffix)
• mg_contains($string, $substring)

Those should return TRUE or FALSE so we can use them as conditions.

[bjendres]

I have implemented those as modifiers. Usage example:

{if $contact.formal_title|mg_startswith:'Dr'}
  Somestuff
{else}
  Otherstuff
{/if}

p.s. thanks to @totten for pointing this option out to me :)