Add default rules for operators

[kana]

For example:

• Typing =:
  1. foo#
  2. foo = #
  3. foo == #
• Typing <BS>:
  1. foo == #
  2. foo = #
  3. foo#

[kana]

• Common operators
  • =
  • +=, -=, ...
  • ==, !=, <=, >=, <, >
  • +, -, *, /, %
  • ++, --
  • <<, >>
  • !, ||, &&
  • |, &, ^, ~
  • ?, :
• Something special
  • ?? (cs)
  • => (ruby)
  • ===, !== (javascript)
  • ==#, ==?, ... (vim)
  • =~, !~, ... (vim)
  • is, isnot, ... (vim)
  • ... and more.

[kana]

Rules for most operators are defined.
But there are several exceptions for each 'filetype'. For example, :symbol in Ruby, <element> in XML, and so forth.

[kana]

• Consider about writing an operator at the beginning/end of a line.

[kana]

Partially resolved by: https://github.com/kana/vim-smartpunc/compare/f607953fb0...7bc9d63e43
There are many rules to define for each filetype. Rules for other filetypes are deferred.