Added json response metadata injection

[leifwalsh]

• If the content-type is application/json, this adds {miniprofiler: {id: ..., link: ..., durationMilliseconds: ...}} to whatever's responded with. Might want to drop the "link" attribute, it's only sort of useful. Might also want to make the "miniprofiler" key configurable in the minuscule chance that it conflicts.
• Also fixed a little string munging bit.

:heart_eyes_cat:

[tcrayford]

released to clojars as 0.4.0

[tcrayford]

and thanks <3