Added ability to specify target of build

[xinslu]

Solves #970.

Simple PR that checks if an optional target is specified in the build command. If yes, matches it and builds only the specific output name.

[codecov]

Codecov Report

Merging #1043 (93be224) into master (a3389b9) will increase coverage by 0.00%.
The diff coverage is 50.00%.

❗ Current head 93be224 differs from pull request most recent head 1786aa3. Consider uploading reports for the commit 1786aa3 to get more accurate results

@@           Coverage Diff           @@
##           master    #1043   +/-   ##
=======================================
  Coverage   45.09%   45.10%           
=======================================
  Files         155      155           
  Lines       62701    62705    +4     
=======================================
+ Hits        28278    28280    +2     
- Misses      34423    34425    +2     

Impacted Files	Coverage Δ
src/bin/tectonic/v2cli.rs	43.20% <50.00%> (+0.07%)	⬆️

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[xinslu]

Hey, I don't mean to be pushy or anything like that. I just wanted to say that this PR is ready for review and let me know if I need to change anything. This is my first OSS contribution, so please excuse my stupidity if I did something wrong.

[pkgw]

@xinslu Sorry, this is all on my end — I've been overwhelmed with work responsibilities lately and am far behind on Tectonic reviews. This sounds like a change that I would like to accept so I aspire to look at it soon! And please feel free to ping me just in case I lose track of things. I try not to do that, but with so much going on it can easily happen.

[pkgw]

I've made a few tweaks here — since I've been taking so long to look at this, I've gone ahead and pushed them to your branch so that I can go ahead and merge once the CI checks pass. Thanks for your contribution!