Improve block parameter parsing

The actual parameter parsing logic in emacs org mode is quite complex [1]. All
we want for now is to handle parameter values containing spaces [2]. Splitting
on ` :` gets us close enough for now. As I'm very opposed to copying 100 lines
of parameter parsing logic just to get exports right let's wait for use cases -
no hurt in gathering requirements as we go.

[1] https://github.com/bzg/org-mode/blob/master/lisp/ob-core.el#L1481
[2] I never ran into such parameters before and wrongly assumed that splitting
on spaces would be enough. boy was i wrong - just look at that massive
function [1]! that's why we can't have nice things!

org/testdata/blocks.html

@@ -40,6 +40,9 @@ a source block with results
 a source block that only exports results
 </pre>
 <pre class="example">
+but the result block is
+</pre>
+<pre class="example">
 an example block with
 multiple lines including
 

org/testdata/blocks.org

@@ -38,6 +38,14 @@ echo a source block that only exports results
 #+RESULTS:
 : a source block that only exports results
 
+#+begin_src bash :cmdline -foo -bar :exports results
+# the code block is not rendered
+echo but the result block is
+#+end_src
+
+#+RESULTS:
+: but the result block is
+
 #+BEGIN_EXAMPLE foo bar baz
 an example block with
 multiple lines including

org/testdata/blocks.pretty_org

@@ -38,6 +38,14 @@ echo a source block that only exports results
 #+RESULTS:
 : a source block that only exports results
 
+#+BEGIN_SRC bash :cmdline -foo -bar :exports results
+# the code block is not rendered
+echo but the result block is
+#+END_SRC
+
+#+RESULTS:
+: but the result block is
+
 #+BEGIN_EXAMPLE foo bar baz
 an example block with
 multiple lines including