Fixed late initialisation and bad state bugs

[alexobviously]

• Fixed issues related to late initialisation in the controller (Programmatically stop and start fails #47)
• Added safety against bad state errors (Unhandled Exception: Bad state: Cannot add new events after calling close #48)
• Resolved a bunch of lints and code style issues

(cc @tamoyal)

[codecov-commenter]

Codecov Report

Merging #51 (8bbb289) into master (01a44ca) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
+ Coverage   96.45%   96.55%   +0.09%     
==========================================
  Files           1        1              
  Lines         141      145       +4     
==========================================
+ Hits          136      140       +4     
  Misses          5        5              

Impacted Files	Coverage Δ
lib/rounded_loading_button.dart	96.55% <100.00%> (+0.09%)	⬆️

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[chrisedg87]

Thanks @alexobviously, great stuff.