Add API for use as -X program or -T CLI tool

[athos]

It would be useful to be able to call some of the functions from the CLI via the -X/-T options.

This PR exposes the following two features as such an API:

• render: Renders the given template
• check: Checks one or more templates

[codecov]

Codecov Report

Merging #10 (685159a) into master (8190b17) will decrease coverage by 0.18%.
The diff coverage is 84.81%.

@@            Coverage Diff             @@
##           master      #10      +/-   ##
==========================================
- Coverage   90.56%   90.37%   -0.19%     
==========================================
  Files          10       11       +1     
  Lines         742      821      +79     
  Branches       37       44       +7     
==========================================
+ Hits          672      742      +70     
- Misses         33       35       +2     
- Partials       37       44       +7     

Impacted Files	Coverage Δ
src/pogonos/api.clj	84.81% <84.81%> (ø)
src/pogonos/output.cljc	87.50% <0.00%> (+3.12%)	⬆️
src/pogonos/error.cljc	88.37% <0.00%> (+4.65%)	⬆️

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[athos]

There is not yet sufficient documentation for this change, but I think it's better to merge the change first than to stall here.