Add print/plot S3 methods for VennDiagram

[stefaneng]

Description

Implement print/plot S3 methods for VennDiagram class.

Closes #26

Checklist

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Example

No grid.draw call required.

library(VennDiagram)
VennDiagram::venn.diagram(list(A = 1:150, B = 121:170), filename = NULL)

[dan-knight]

Same as previous version number.

[dan-knight]

Should this be 1.8.0 as this is technically new functionality?

[dan-knight]

Very cool quality of life improvement. Could we add some unit tests for the different cases in the S3 function?

[stefaneng]

Very cool quality of life improvement. Could we add some unit tests for the different cases in the S3 function?

I'm not quite sure how to test this. Any suggestions?

[dan-knight]

I'm not quite sure how to test this. Any suggestions?

Given the current end-to-end test cases, do the changes you made here affect them as written? In other words, if this code was incorrect, would it make the existing test cases fail?

[stefaneng]

I'm not quite sure how to test this. Any suggestions?

Given the current end-to-end test cases, do the changes you made here affect them as written? In other words, if this code was incorrect, would it make the existing test cases fail?

All the old tests still work. No, it would not make the tests fail since there old tests don't look at class at all and there is no print or plot method called at all.

[dan-knight]

No, it would not make the tests fail since there old tests don't look at class at all and there is no print or plot method called at all.

Right. It might just be good to have some code that explicitly calls the S3 methods you defined here. Also, maybe a test case that asserts is('VennDiagram')?