Fix for a couple of bugs.

Fixes #2, the README previously said to run set fmt-onsave false, which was missing the plugin prefix. The working command is set autofmt.fmt-onsave false

Fixes #4

There were a couple of small bugs causing the functionality not to work.

• Having RegisterCommonOption inside the init() funtion stops the default value from taking effect.
  • Credit to the Micro Golang plugin for helping me figure this out.
• Similar to the first fix, the tryFmt function was checking fmt-onsave instead of autofmt.fmt-onsave.
• Previously MakeCommand was mapping fmt to tryFmt, which had the side effect that fmt would not work if the fmt-onsave option was disabled. I fixed this by adding a new function, fmtCmd, which is the same as tryFmt but without the config check.