Generate compilation database for clang by default

Add the usual vim plugin load protection

Tweak how configs and targets are parsed and handled

Clean-up project name before setting it, and do proper error reporting.

Don't complain on startup if no UE branch is found.

Support command bang for building.

Add help info on statuslines.

Use the correct documentation extension.

Add documentation.

Refactor the build system invocation commands.

Now we have proper knowledge of the projects inside a codebase ("branch"). The
plugin should correctly parse configuration names, find the correct module to
build based on the configuration, and so on.

Also, added support for generating the clang compilation database.

Don't change the vimcrosoft solution if it's not needed. Fix indenting.

Initial commit.