Launcher plugin documentation, CRAN feedback, and mirai compat

crew 0.0.5

• Write a call() method to make it easier to write custom launcher plugins.
• Add robust documentation and tests for custom launcher plugins.
• Use as.character() to make sure the list argument supplied to rm() is safe. Fixes a test that was failing on R-devel with crew version 0.0.4.
• Remove the seconds_poll_high and seconds_poll_low fields and arguments for compatibility with mirai development and release versions.
• Hard-code the async_dial field in the router for compatibility with mirai development and release versions.