Add action to delete projects from helm-projectile-switch-project

It's annoying to delete projects one by one. This action is added to
solve that problem.
Basic support of Rust’s Cargo projects
Merge pull request #476 from vhbit/rust-cargo

Basic support of Rust’s Cargo projects
Merge pull request #475 from tuhdo/master

Add action to delete projects from helm-projectile-switch-project
Add helm-projectile-find-file-in-known-projects

Also refactor projectile-find-file-in-known-projects and separate code
that returns files in all projects into a function for reusability.
Merge pull request #477 from tuhdo/master

Add helm-projectile-find-file-in-known-projects
Update README and CHANGELOG
Merge pull request #481 from tuhdo/master

Update README and CHANGELOG
Fix erroneous call to non-existent function
Merge pull request #482 from tuhdo/master

Fix erroneous call to non-existent function
Merge pull request #474 from YorkZ/helm-switch-project

Performance optimization and eliminate boilerplate code
Cleanup `helm-projectile-file-actions' definition

Previously, `helm-projectile-file-actions' was defined by copying and pasting
the actions defined in `helm-source-ffiles' first, then modifying a few entries
in the copied list. This is boilerplate and not flexible. For example, if a
function name gets changed in Helm, this list will have to be changed
accordingly.

By adding the function `helm-projectile-hack-actions', we are now able to hack
directly on a dynamic copy of the original list in Helm, so that function name
changes in Helm will less likely to break helm-projectile.