Add iferr integration (#15)

* feat(iferr): add to installer & config

* feat(iferr): add first implementation

* docs(iferr): add

lua/gopher/iferr.lua

@@ -0,0 +1,21 @@
+local c = require("gopher.config").config.commands
+local u = require "gopher._utils"
+
+---Add iferr declaration
+---That's Lua of vimscript implementation of:
+---github.com/koron/iferr
+return function()
+  local boff = vim.fn.wordcount().cursor_bytes
+  local cmd = (c.iferr .. " -pos " .. boff)
+  local data = vim.fn.systemlist(cmd, vim.fn.bufnr "%")
+
+  if vim.v.shell_error ~= 0 then
+    u.notify("command " .. cmd .. " exited with code " .. vim.v.shell_error, "error")
+    return
+  end
+
+  local pos = vim.fn.getcurpos()[2]
+  vim.fn.append(pos, data)
+  vim.cmd [[silent normal! j=2j]]
+  vim.fn.setpos(".", pos)
+end