Add optional threshold for budgets

[wenhaogoh]

Closes #109

[codecov-io]

Codecov Report

Merging #128 into master will decrease coverage by 1.29%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #128      +/-   ##
============================================
- Coverage     28.72%   27.42%   -1.30%     
+ Complexity      161      158       -3     
============================================
  Files            84       85       +1     
  Lines          1309     1338      +29     
  Branches         97      100       +3     
============================================
- Hits            376      367       -9     
- Misses          906      946      +40     
+ Partials         27       25       -2     

Impacted Files	Coverage Δ	Complexity Δ
...dress/logic/commands/main/CreateBudgetCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...in/java/seedu/address/logic/parser/ParserUtil.java	34.09% <0.00%> (-4.38%)	6.00 <0.00> (ø)
...rser/mainpageparser/CreateBudgetCommandParser.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...c/main/java/seedu/address/model/budget/Budget.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...ain/java/seedu/address/model/budget/Threshold.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
.../java/seedu/address/storage/JsonAdaptedBudget.java	0.00% <0.00%> (-34.79%)	0.00 <0.00> (-2.00)
src/main/java/seedu/address/model/budget/Name.java	0.00% <0.00%> (-10.00%)	0.00% <0.00%> (-1.00%)

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[yu-ming-chen]

LGTM