Never overwrite other mappings

Use `maparg()` to ensure we don't ever overwrite existing mappings.
There's also `mapcheck()` which will find partial matches, but I think
the more stricter check is appropriate here (`mapcheck()` is good for
finding prefix collisions which could make mappings slow).

Signed-off-by: Greg Hurrell <greg@hurrell.net>
Use vim's autoload feature

By autoloading, command-t's startup time is reduced from 66ms to 0ms
on my system (see vim --startuptime).

Originally submitted as:

  https://github.com/wincent/Command-T/pull/81

Signed-off-by: Greg Hurrell <greg@hurrell.net>
Update docs with info about autoload optimization

Signed-off-by: Greg Hurrell <greg@hurrell.net>