Add slash to focus on command input

[jeffsieu]

Pressing slash will now focus on the command box input

[codecov-commenter]

Codecov Report

Merging #56 (eb59ba8) into master (737d179) will decrease coverage by 0.25%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master      #56      +/-   ##
============================================
- Coverage     58.47%   58.22%   -0.26%     
  Complexity      417      417              
============================================
  Files            90       90              
  Lines          1616     1623       +7     
  Branches        166      166              
============================================
  Hits            945      945              
- Misses          636      643       +7     
  Partials         35       35              

Impacted Files	Coverage Δ
src/main/java/seedu/address/ui/CommandBox.java	0.00% <0.00%> (ø)
src/main/java/seedu/address/ui/MainWindow.java	0.00% <0.00%> (ø)

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