helm-projectile.el (helm-projectile-define-key): performance optimization

Because "list" in Emacs Lisp is singly-linked list, frequently adding to the end
of a list using `add-to-list' can be slow. For this reason, it's better to add
to the front of a list, and just do a one-time reverse after adding all the
items.
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
projectile.el (projectile-project-vcs): minor cleanup
Refactor `projectile-vc' to allow being used in `helm-projectile-vc'

Added the optional parameter `project-root' for `projectile-vc', so that the
function can be called from `helm-projectile-vc', to eliminate boilerplate code.
Merge pull request #474 from YorkZ/helm-switch-project

Performance optimization and eliminate boilerplate code
Change key binding compile project action

C-c is common prefix that should be reserved for users. We should bind
compile action to something else, like M-c.
Remove an extra space
Update the MELPA urls
Fix error when Projectile fails to get sub-project files

When external command for getting sub-project files fails, it should not
break Projectile. Simply fall back to the default behavior.
Merge pull request #484 from tuhdo/master

Change key binding compile project action, fix external command error and add new action to create virtual directory
Add melpa badges