(TK-11) Make custom middlewares possible in the nrepl-service

With this change, one can enable custom middlewares for the nrep.
For example to make the LightTable nrepl connection work:
Add [lein-light-nrepl "0.0.4"] to your project dependencies.
Configure your nrepl-service like this:

[nrepl]
port = 12345
enabled = true
middlewares = [lighttable.nrepl.handler/lighttable-ops]
add test middleware to test nrepl-service middleware support
Add a test for array-based configuration, add docs

Some of our other supported config formats support arrays for the
values of the data, but it will generally be made available as
arrays of strings.  This commit adds a minor tweak so that
we will call `symbol` on these strings before using them, and
adds a test validating that this type of configuration will
work properly.

The commit also adds documentation for the new setting.
Merge pull request #1 from cprice404/feature/nrepl-middleware

Add a test for array-based configuration, add docs
Merge pull request #147 from exi/feature/nrepl-middleware

(TK-11) Make custom middlewares possible in the nrepl-service