feat(installer): add support for symbolic icons

[fcastilloec]

See https://developer.gnome.org/ThemedIcons/ for more info on symbolic icons. We currently only support SVG for this type of icons, since it's the preferred one.

[codecov]

Codecov Report

Merging #70 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #70   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines          198       210   +12     
=========================================
+ Hits           198       210   +12     

Impacted Files	Coverage Δ
src/installer.js	100.00% <100.00%> (ø)
src/dependencies.js	100.00% <0.00%> (ø)
src/readmetadata.js	100.00% <0.00%> (ø)

---

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 759c691...2e3eac4. Read the comment docs.

[malept]

One nit but otherwise

[fcastilloec]

@malept When do you think it'll be a good idea to release v1.0.0? The code and API have become stable enough, and other modules are depending on this.
I have some other private modules and have been wondering when to release v1 until I found this

[malept]

I think 1.0.0 will be released when everything in the TODOs (and follow ups) for #71 are done, plus removing the deprecated spawn export.