Prototype of use third party css and js directory

To avoid duplication of css and js directory we can rely in this files being provide by a third party. This is a good option because if our css or js files need to change we only need to push the changes to one repository. The "problem" is that developers will need to be online to properly preview their changes. This "problem" is solved by this PR.

## Novice Use (Require to be online for properly preview)

~~~
$ make -B all
pandoc \
    -s \
    -t html5 \
    --template=_layouts/lesson \
    -V SWCCOMMONADDRESS=http://software-carpentry.org/v5 \
    -o ../index.html index.md
$ grep css ../index.html 
  <link rel="stylesheet" type="text/css" href="http://software-carpentry.org/v5/css/swc.css" />
~~~

## Intermediate Use

~~~
$ LOCALSWCFILES=. make -B all
pandoc \
    -s \
    -t html5 \
    --template=_layouts/lesson \
    -V SWCCOMMONADDRESS=. \
    -o ../index.html index.md
$ grep css ../index.html     
  <link rel="stylesheet" type="text/css" href="./css/swc.css" />
~~~

To avoid type `LOCALSWCFILES=.` every time you can add

~~~
export LOCALSWCFILES=.
~~~

into your `~/.bashrc`.

In case the contributor want to keep only one copy of `css` and `js` directories this is also possible. Just use `LOCALSWCFILES=../relative/path`.