create a tangld-build function that tangles org-source files into their targets

[aldrichtr]

Overview

loop through the org-mode files in the source directory, tangling each one.

tangld-build needs a list of one or more org files and then for each file in the list, tangld-build runs org-bable-tangle on it and then moves to the next file.

At the most basic level, the src blocks in the file define the options and the target that are given to the org-babel-tangle-file function. If desired, the user can create other src-blocks in other files that can add additional functions and/or data to the current file's src-blocks. This is done by loading the library-of-babel which is documented on the org site.

Secondly, by default we shouldn't tangle a file that hasn't changed since the last tangle. This should help reduce "build time" and will also keep file timestamps consistent with changes for the most part.

Lastly, the user must have the ability to include and exclude any file they want from the list that tangld-build uses.

Tasks

• Produce a filtered list of files to be processed  #21
• Determine if a file has been modified since the last time it was tangled #7
• ingest org-mode files in lib directories into the library of babel for use in build #5
• Wrap the org-babel-tangle function with options from the tangld project configuration #20