Add show method for MIME"text/plain"

[giordano]

Add a show method to print the value and unit parts in case they have custom representations with MIME text/plain

[coveralls]

Coverage increased (+1.8%) to 79.067% when pulling 95af0c9 on giordano:show-mime into 82f8496 on ajkeller34:master.

[codecov]

Codecov Report

Merging #213 into master will decrease coverage by 0.39%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master     #213     +/-   ##
=========================================
- Coverage   79.95%   79.56%   -0.4%     
=========================================
  Files          14       14             
  Lines         998     1003      +5     
=========================================
  Hits          798      798             
- Misses        200      205      +5

Impacted Files	Coverage Δ
src/display.jl	78.37% <0%> (-12.25%)	⬇️

---

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 82f8496...112c9f7. Read the comment docs.

[codecov]

Codecov Report

Merging #213 into master will decrease coverage by 0.09%.
The diff coverage is 60%.

@@            Coverage Diff            @@
##           master     #213     +/-   ##
=========================================
- Coverage   79.95%   79.86%   -0.1%     
=========================================
  Files          14       14             
  Lines         998     1003      +5     
=========================================
+ Hits          798      801      +3     
- Misses        200      202      +2

Impacted Files	Coverage Δ
src/display.jl	86.48% <60%> (-4.14%)	⬇️

---

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 82f8496...95af0c9. Read the comment docs.

[ajkeller34]

Merged into rigetti/Unitful.jl. will merge here

[codecov-io]

Codecov Report

Merging #213 into master will decrease coverage by 0.09%.
The diff coverage is 60%.

@@            Coverage Diff            @@
##           master     #213     +/-   ##
=========================================
- Coverage   79.95%   79.86%   -0.1%     
=========================================
  Files          14       14             
  Lines         998     1003      +5     
=========================================
+ Hits          798      801      +3     
- Misses        200      202      +2

Impacted Files	Coverage Δ
src/display.jl	86.48% <60%> (-4.14%)	⬇️

---

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 82f8496...95af0c9. Read the comment docs.