feat: add custom format specs for JSON

[3Hren]

@shaitan, part 1: functionality.

[3Hren]

Performance regression:

Before:

⇒  ./blackhole-benchmarks
Run on (4 X 1000 MHz CPU s)
2016-06-21 16:56:55
Benchmark                                    Time(ns)    CPU(ns) Iterations
---------------------------------------------------------------------------
formatter.json                                    881        875     791649                1115.91k items/s
formatter.json[route]                            1107       1101     628067                887.084k items/s
formatter.json[route + attr: 1]                  1266       1258     553014                776.236k items/s
formatter.json[route + attr: 5]                  2065       2050     334271                476.418k items/s
formatter.json[route + attr: 5 + unique]         2752       2706     265371                360.829k items/s

After:

⇒  ./build/blackhole-benchmarks --benchmark_filter=formatter\.json
Run on (4 X 1000 MHz CPU s)
2016-08-22 14:58:29
Benchmark                                  Time(ns)    CPU(ns) Iterations
-------------------------------------------------------------------------
formatter.json                                 1013       1009     682594                967.397k items/s
formatter.json[route]                          1231       1228     554825                795.182k items/s
formatter.json[route + attr: 1]                1454       1448     491111                674.306k items/s
formatter.json[route + attr: 5]                2362       2358     300827                414.183k items/s
formatter.json[route + attr: 5 + unique]       2850       2846     240198                343.118k items/s

[3Hren]

Closes #147.