add motion_specific_args

[fjchen7]

The motivation behind this PR is to add more fine-grained options to set motion.

For example, for me it is more comfortable to exclude the match when using f/F in operation mode, and my settings will be like

require("flit").setup {
  keys = { f = "f", F = "F" },
  labeled_modes = "vno",
  motion_specific_args = {
    o = {
      f = { offset = -1 },
      F = { offset = 1 },
    },
  }
}

Related issue: #7