Add TagFindCommand

[akgrenSoar]

Closes #67

Add TagFindCommand changelog:

• Add TagFindCommand
• Update Userguide
• Add relevant unit tests

Other Changes:

• TagName now allow spaces
• Commands in UserGuide are now in lowercase

[codecov-io]

Codecov Report

Merging #65 into master will increase coverage by 0.61%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master      #65      +/-   ##
============================================
+ Coverage     69.30%   69.92%   +0.61%     
- Complexity      426      442      +16     
============================================
  Files            77       80       +3     
  Lines          1362     1390      +28     
  Branches        139      142       +3     
============================================
+ Hits            944      972      +28     
  Misses          387      387              
  Partials         31       31              

Impacted Files	Coverage Δ	Complexity Δ
...main/java/seedu/address/commons/core/Messages.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...a/seedu/address/logic/commands/TagListCommand.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
src/main/java/seedu/address/model/tag/TagName.java	90.00% <ø> (ø)	7.00 <0.00> (ø)
...a/seedu/address/logic/commands/TagFindCommand.java	100.00% <100.00%> (ø)	5.00 <5.00> (?)
...java/seedu/address/logic/parser/ProjactParser.java	100.00% <100.00%> (ø)	14.00 <0.00> (+1.00)
...edu/address/logic/parser/TagFindCommandParser.java	100.00% <100.00%> (ø)	3.00 <3.00> (?)
...ss/model/tag/TagNameContainsKeywordsPredicate.java	100.00% <100.00%> (ø)	7.00 <7.00> (?)

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[siqiang-ng]

LGTM other than the above comment to clarify

[yeo-kexin]

Resolve merge conflicts in TagBuilder.java

[yeo-kexin]

LGTM