Re-work Vimrunner::Client API

[mudge]

Based on discussion in #12, reconsider the API provided by Vimrunner::Client to:

• Provide raw access to the --remote-send and --remote-expr interfaces;
• Allow for the execution of commands and capture their output (perhaps simpler stated as an interface for ex commands);
• Provide an abstraction that simulates raw user keypresses (replacing type with something presumably feedkeys()-powered);
• Provide an abstraction between the various modes: normal, insert, visual, visual block, etc.