Keep add button visible

[Katyadelarosa]

Seems like a good idea to have an option to keep the Add button always visible, because when you start typing it disappears until you go to the next line.

[codecov-io]

Codecov Report

Merging #153 into master will increase coverage by 0.47%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
+ Coverage   81.66%   82.13%   +0.47%     
==========================================
  Files          15       15              
  Lines         349      347       -2     
  Branches       50       49       -1     
==========================================
  Hits          285      285              
+ Misses         64       62       -2

Impacted Files	Coverage Δ
src/model/index.js	58.44% <0%> (+1.47%)	⬆️

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[brijeshb42]

@Katyadelarosa What happens to the text that is already there in that block that has the (+) button?

[Katyadelarosa]

@brijeshb42 Nothing, it doesn't affect existing content, but I just pushed a fix for the position of the (+) button, it was not updating when moving between blocks with content

[brijeshb42]

@Katyadelarosa Just got a chance to look at this and it seems like your PR changes the default behavior and makes the (+) button always be visible by default. Your changes should not change the default behavior and should only be applied when enabled through prop. Let me know if you can take a look. Otherwise, I can merge your PR and apply the required changes after that and then publish a new version.