Make the help command useful

[zhiayang]

Closes #136, closes #160.

No need to print usage because:

1. the box is too small
2. just have a link to the UG

[codecov-io]

Codecov Report

Merging #181 into master will increase coverage by 1.88%.
The diff coverage is 54.86%.

@@             Coverage Diff              @@
##             master     #181      +/-   ##
============================================
+ Coverage     41.91%   43.79%   +1.88%     
- Complexity      762      826      +64     
============================================
  Files           134      135       +1     
  Lines          3519     3715     +196     
  Branches        483      503      +20     
============================================
+ Hits           1475     1627     +152     
- Misses         1872     1915      +43     
- Partials        172      173       +1     

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/chopchop/MainApp.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
src/main/java/chopchop/commons/util/Strings.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
src/main/java/chopchop/logic/commands/Command.java	35.29% <ø> (ø)	3.00 <0.00> (ø)
...ava/chopchop/logic/commands/MakeRecipeCommand.java	12.24% <0.00%> (-0.80%)	2.00 <0.00> (ø)
...main/java/chopchop/logic/commands/QuitCommand.java	33.33% <0.00%> (-33.34%)	2.00 <0.00> (ø)
...main/java/chopchop/logic/commands/RedoCommand.java	25.00% <0.00%> (-15.00%)	2.00 <0.00> (ø)
...hop/logic/commands/StatsIngredientDateCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...hopchop/logic/commands/StatsRecipeDateCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...hop/logic/commands/StatsRecipeMostMadeCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...main/java/chopchop/logic/commands/UndoCommand.java	25.00% <0.00%> (-15.00%)	2.00 <0.00> (ø)
... and 27 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3a2cdc6...42cc1f6. Read the comment docs.