Add flamegraph_mode config option and query string parameter

[evanleck]

With this change, the :mode passed to StackProf.run can be customized just like the sample rate.

[SamSaffron]

This looks great to me, can you also add a note to the README here?

@OsamaSayegh can you double check?

[codecov-commenter]

Codecov Report

Merging #508 (43c0e9c) into master (3f47ae9) will decrease coverage by 0.26%.
The diff coverage is 20.00%.

@@            Coverage Diff             @@
##           master     #508      +/-   ##
==========================================
- Coverage   87.51%   87.25%   -0.27%     
==========================================
  Files          18       18              
  Lines        1258     1263       +5     
==========================================
+ Hits         1101     1102       +1     
- Misses        157      161       +4     

Impacted Files	Coverage Δ
lib/mini_profiler/profiler.rb	83.19% <0.00%> (-0.72%)	⬇️
lib/mini_profiler/config.rb	88.15% <100.00%> (+0.15%)	⬆️

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[OsamaSayegh]

Looks good to me! Just need a note about this in the README like Sam mentioned. Thank you for this!

[evanleck]

@SamSaffron @OsamaSayegh my pleasure! I've added a note to the README as requested :)