Allow excluding links from the internal_links checker.

Links to be excluded can be specified as a list of regular
expressions in config[:checks][:internal_links][:exclude].
Set explicit UTF-8 encoding in nanoc.yaml by default (fixes #239)
Merge branch 'release-3.6.x'
Merge pull request #262 from ddfreyne/feature-default-encoding-utf-8

Set explicit UTF-8 encoding in nanoc.yaml by default
Fix failing missing default deploy configuration test
Fix test failing due to changed deploy command message
Move finding Checks filename out of constructor
Do not move files out of `content/` (fixes #271)

This is not as efficient as it could be, because moving files out of the
temporary directory into `output/` is perfectly fine. Copying binary
items into the temporary directory just so they can be safely moved is
also not efficient either, though.

And yes, @bobthecow was right all along.
Merge pull request #272 from ddfreyne/bug-move-files-out-of-content

Do not move files out of `content/` (fixes #271)
Merge pull request #242 from remko/master

Allow excluding links from the internal_links checker
Copy content before passing it to Kramdown

Kramdown 1.0.0 calls #encode! on the source string, but the one passed
by nanoc is frozen.
Fix forwardable methods in 1.8.x
Merge pull request #278 from bobthecow/vintage-forwardable

Fix forwardable methods in 1.8.x
Merge pull request #276 from cdlm/fix-kramdown-v1

Copy content before passing it to kramdown, for compatibility with 1.0.0
Add license to gemspec to stop Rubygems from complaining
Fix fun/run typo (fixes #280)

Thanks to @gjtorikian, nanoc is now 100% less fun. :(
Simplify external links test case
Fallback to GET when HEAD is not allowed (#274)
Use ExternalLinks obj in ExternalLinks test
Do not duplicate content before passing it to kramdown (fixes #282)

The content passed to kramdown was duplicated because kramdown 1.0.0
modifies the encoding in-place, crashing nanoc. kramdown 1.0.1 fixed
this issue. See #276, #277.