Added ability to match do/end pairs plus fixed various bugs.

[stevemadere]

New features:

• Matches do/end pairs

Bugs fixed:

• Works better when the cursor is on one of these: {[()]}
• When the cursor is not on a supported matched end point
  or a match cannot be found, it used to jump to the beginning
  of the line. Now it stays put.
• Uses the safer normal! instead of normal at various places.

Flaws:

• Hacky use of 'normal! mZ' may screw up your marks

[stevemadere]

This is the hacky use of mZ.
It is a problem in that if the user had marked something else with mZ, this will override that and
would likely cause extreme confusion and frustration.
I am open to suggestions of how else to return to the exact location (line and column) at which user started.