allow initial callback changes

[ChrisRackauckas]

SciML/DifferentialEquations.jl#194

[ChrisRackauckas]

To make this not cause a performance hit I changed the default initialization: SciML/DiffEqBase.jl@32d413d

[coveralls]

Coverage increased (+0.01%) to 76.766% when pulling e3fa817 on initial_callbacks into 0d0b6b2 on master.

[codecov]

Codecov Report

Merging #213 into master will decrease coverage by 0.03%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
- Coverage   76.75%   76.72%   -0.04%     
==========================================
  Files          61       61              
  Lines       21424    21435      +11     
==========================================
+ Hits        16444    16445       +1     
- Misses       4980     4990      +10

Impacted Files	Coverage Δ
src/solve.jl	87.35% <16.66%> (-5.29%)	⬇️

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[coveralls]

Coverage decreased (-0.03%) to 76.72% when pulling 3263f4c on initial_callbacks into 0d0b6b2 on master.