Provide an easy way to "adopt a dotfile" into tangld

[aldrichtr]

Provided that tangld is already initialized and configured, make it easy to "move this dotfile into tangld". For example, I already have tangld setup in ~/.tangld and I have a ~/.tmux.conf file in my home directory that I want to bring under tangld. Adopting this file means:

• create an org source file in ~/.tangld/src with an appropriate name (in this case 'tmux.org' seems fitting)
• add appropriate header information to the org file, such as a title, tangle target info, comment options etc
• add an org heading to the org file
• add a begin_src block
• copy the contents of ~/.tmux.conf into tmux.org
• add a end_src block
• move or rename the original source file