Add an overload to render() that accepts a context and a stream

[KitsuneRal]

There's a two-argument render() overload for basic_data<>, making one for basic_context<> in the same way is pretty obvious.

[codecov-io]

Codecov Report

Merging #21 into master will increase coverage by 0.35%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   99.21%   99.57%   +0.35%     
==========================================
  Files           2        2              
  Lines        1410     1404       -6     
==========================================
- Hits         1399     1398       -1     
+ Misses         11        6       -5

Impacted Files	Coverage Δ
tests.cpp	99.28% <ø> (+0.57%)	:arrow_up:
mustache.hpp	100% <100%> (ø)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 9dce078...c71aa5c. Read the comment docs.

[KitsuneRal]

Peeking at how rendering to a stream is tested now, made a replica that accepts a context.

[KitsuneRal]

I took the liberty and rebased the PR on the latest master.

[KitsuneRal]

That's what happens when not running tests after rebasing... Fixed.