Introduce common Poedit headers. Related #28
Add X-Poedit-SearchPathExcluded-0 header support

As seen at https://github.com/robincornett/simple-listings-genesis/commit/ab3642f785f4324942aeb0323061bd8ace341812#diff-dfa28679ad06a5c3fe51c70a4b389d4fR27
Fix keywords list common header default value

I spent considerable time looking into what this deciphers as, and most themes and
plugins have it wrong. The string that this commit replaces included deprecated
functions that no-one should be using, and didn't include the escaping functions.
Update unit tests for X-Poedit-SearchPathExcluded-0 header
Merge pull request #32 from GaryJones/search-path-exclude

Add X-Poedit-SearchPathExcluded-0 header support
Merge pull request #33 from GaryJones/keywordslist-common-headers

Fix keywords list common header default value
Add unit tests for common Poedit headers.
Minor grammar/punctuation cleanup in unit test messages.
Add an option to print the results of the addtextdomain task to stdout
instead of updating files.

Toggled by adding --dry-run as a CLI arg.
Add docs for the new potHeader options.
Use the OS EOL character in addtextdomain tests.
Flip 'actual' and 'expected' arguments in makepot unit tests to match the docs.
Add unit tests to be sure expanded Poedit headers can be overridden.
Split the readme into separate files to make it easier to digest.
Prepend a header in the custom headers test to verify that the order doesn't matter.
Mention the addtextdomain --dry-run switch in the docs.
Update the title of the changelog to be more clear what it's referencing.
Bump to version 0.4.8