Add support for constant variables in viewer

[eliascarv]

closes #326

[codecov-commenter]

Codecov Report

Attention: 32 lines in your changes are missing coverage. Please review.

Comparison is base (ee210e2) 0.57% compared to head (d5c4fdd) 0.51%.

Files	Patch %	Lines
ext/viewer.jl	0.00%	29 Missing :warning:
ext/colors.jl	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #331 +/- ## ========================================= - Coverage 0.57% 0.51% -0.07% ========================================= Files 6 6 Lines 175 196 +21 ========================================= Hits 1 1 - Misses 174 195 +21 ```

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