First version of the inline plugin.

In some circumstances it is useful to copy the contents of a file into another file. This can be especially useful when trying to reduce the perceived load time of a page by optimising the critical render path. Critical JS and CSS can now easily be inlined into the index.html page using this plugin.

Example of usage:

<@inline file=“../libs/mylibrary/src/mylibrary/FileToInline.js” @/>

The ‘file’ argument will be relative to the file that includes the ‘inline’ tag.
Throw an error if the inline file is not found
Add some unit tests
Improve the file not found error message.
Tidy up the unit tests
Add test for subfolder include
Merge pull request #1053 from stevesouth/inline-plugin

Inline plugin