Addresses 914; capture blocks in macros now write to temporary output buffer

[noahlange]

Summary

Proposed change: Fixes a bug that led the capture block compiler to write to the main buffer instead of the temporary one. Previously, the following code would (incorrectly) write "foobar".

{%- macro foo(bar) -%}
  {%- set test -%}
    foo
  {%- endset -%}
  {{ bar }}{{ test }}
{%- endmacro -%}
{{ foo("bar") }}

Now it writes "barfoo."

Closes #914.

Checklist

I've completed the checklist below to ensure I didn't forget anything.
( I accidentally hit enter before finishing the checklist.)

• Proposed change helps towards purpose of this project.
• Documentation is added / updated to describe proposed change.
• Tests are added / updated to cover proposed change.
• Changelog has an entry for proposed change (if user-facing fix or feature).

[noahlange]

This also appears to solve #972.

[fdintino]

This looks great, thanks!