Extension work

Hi,

Getting closer :)

I've got the zotonic context into the templates now too.. and I have a first custom tag working,
so now it's on to port over the remaining tags and features..

The tests on the ErlyDTL side is not really keeping up wrg to the extension stuff though..
Had a hard time coming up with contrived examples to test for.. :p

Will start sketching on a howto-extend-erlydtl kind of thing..


The compilation takes a couple of ms longer compared to our forked ErlyDTL version.. but that's ok, I think :)
(it's the rendering that is most important anyhow..)

```
(zotonic001@precise32)3> ztl_tests:test().
src/ztl/ztl_tests.erl:133:<0.616.0>: -- engine zerlydtl
src/ztl/ztl_tests.erl:138:<0.616.0>: compiling: simple test: 0.026 s
src/ztl/ztl_tests.erl:150:<0.616.0>: rendering: simple test: 0.000 s
src/ztl/ztl_tests.erl:87:<0.616.0>: test case: simple test: 0.027 s
src/ztl/ztl_tests.erl:133:<0.616.0>: -- engine ztl
src/ztl/ztl_tests.erl:138:<0.616.0>: compiling: simple test: 0.028 s
src/ztl/ztl_tests.erl:150:<0.616.0>: rendering: simple test: 0.000 s
src/ztl/ztl_tests.erl:87:<0.616.0>: test case: simple test: 0.029 s
src/ztl/ztl_tests.erl:133:<0.616.0>: -- engine zerlydtl
src/ztl/ztl_tests.erl:138:<0.616.0>: compiling: auto id: 0.025 s
src/ztl/ztl_tests.erl:150:<0.616.0>: rendering: auto id: 0.000 s
src/ztl/ztl_tests.erl:87:<0.616.0>: test case: auto id: 0.027 s
src/ztl/ztl_tests.erl:133:<0.616.0>: -- engine ztl
src/ztl/ztl_tests.erl:138:<0.616.0>: compiling: auto id: 0.028 s
src/ztl/ztl_tests.erl:150:<0.616.0>: rendering: auto id: 0.000 s
src/ztl/ztl_tests.erl:87:<0.616.0>: test case: auto id: 0.028 s
src/ztl/ztl_tests.erl:133:<0.616.0>: -- engine zerlydtl
src/ztl/ztl_tests.erl:138:<0.616.0>: compiling: url tag: 0.025 s
src/ztl/ztl_tests.erl:150:<0.616.0>: rendering: url tag: 0.000 s
src/ztl/ztl_tests.erl:87:<0.616.0>: test case: url tag: 0.026 s
src/ztl/ztl_tests.erl:133:<0.616.0>: -- engine ztl
src/ztl/ztl_tests.erl:138:<0.616.0>: compiling: url tag: 0.028 s
src/ztl/ztl_tests.erl:150:<0.616.0>: rendering: url tag: 0.000 s
src/ztl/ztl_tests.erl:87:<0.616.0>: test case: url tag: 0.030 s
  All 6 tests passed.
ok
```