Version 7.4.0

Mildly breaking changes

These changes are technically breaking changes, but they should only affect advanced users.

Restore compatibility with clustermq (#898). Suggest version >= 0.8.8 but allow 0.8.7 as well.
Ensure drake recomputes config$layout when knitr reports change (#887).
Do not rehash large imported files every make() (#878).
Repair parsing of long tidy eval inputs in the DSL (#878).
Clear up cache confusion when a custom cache exists adjacent to the default cache (#883).
Accept targets as symbols in r_drake_build().
Log progress during r_make() (#889).
Repair expose_imports(): do not do the environment<- trick unless the object is a non-primitive function.
Use different static analyses of assign() vs delayedAssign().
Fix a superfluous code analysis warning incurred by multiple file_in() files and other strings (#896).
Make ignore() work inside loadd(), readd(), file_in(), file_out(), and knitr_in().

Add experimental support for URLs in file_in() and file_out(). drake now treats file_in()/file_out() files as URLS if they begin with "http://", "https://", or "ftp://". The fingerprint is a concatenation of the ETag and last-modified timestamp. If neither can be found or if there is no internet connection, drake throws an error.
Implement new memory management strategies "unload" and "none", which do not attempt to load a target's dependencies from memory (#897).
Allow users to give each target its own memory strategy (#897).
Add drake_slice() to help split data across multiple targets. Related: #77, #685, #833.
Introduce a new drake_cache() function, which is now recommended instead of get_cache() (#883).
Introduce a new r_deps_target() function.
Add RStudio addins for r_make(), r_vis_drake_graph(), and r_outdated() (#892).