Support method chaining

[jayqi]

It's common practice in object-oriented programming to support method chaining.

Unfortunately, we have several methods that should work this way (under typical OOP implementations) but return invisible(NULL) instead of invisible(self). One example of this is $set_package, which means you can't do something like this:

library(pkgnet)
FunctionReporter$new()$set_package('baseballstats')$pkg_graph
#> INFO [2018-12-18 22:17:36] Checking installed packages...
#> INFO [2018-12-18 22:17:36] Found 'baseballstats' in installed packages.
#> INFO [2018-12-18 22:17:36] Resetting cached network information...
#> NULL

We should go through and change all public methods that don't return something specific to return invisible(self).

^{Created on 2018-12-18 by the reprex package (v0.2.1)}