Add dialyzer.build task

[christhekeele]

Adding the ability to explicitly build the plts without running any dialyzer checks would be helpful for warming up caches in CI environments. This would be especially useful if your CI doesn't allow for configuring test task timeout but is more generous during the installation phase; or if it allows for extending task timeout but you want to know if for some reason your dialyzer checking time takes forever (independent of the plt build time).

It's also the logical inverse of the dialyzer.clean task proposed in #1, and I love symmetry.

This could also be called dialyzer.compile.

Alternately it could instead be registered under the compile namespace to allow this to occur automatically alongside other compilation tasks when mix compile is run, perhaps under the name compile.plts or compile.dialyzer. I'm inclined to avoid that though since that could unexpectedly make project compile times skyrocket in environments where dialyzer users aren't anticipating it.