Merge pull request #15 from josh/master

Loosen Gemoji restriction
add readme examples section
Merge pull request #16 from jch/readme-examples

What is the github filter chain?
readme is oss manifesto friendly, add changelog [skip ci]
Fixed typos
Merge pull request #17 from pborreli/typos

Fixed typos [skip ci]
tryna is a real word! [skip ci]

See #17, but we'll go the boring plain english route.
Use correct OSS Manifesto link.
Merge pull request #18 from wickedshimmy/patch-1

Use correct OSS Manifesto link. [skip ci]
Added an example of a custom filter.
Fixed typo in example.
Added AbsoluteSourceFilter for changing relative src to Fully Qualified

This is useful if an image is root relative but should really be going through
a cdn, or if the content for the page assumes the host is known i.e. scraped
webpages and some RSS feeds.
Extending example copy edits.
Made AbsoluteSourceFilterTest exceptions include their filter class name.

This should help identify what contexts are required for the filter.
Test have also been updated to make sure exceptions are raised when
required contexts are missing and that the exception message includes
the name of the class where the required context is missing.
Corrected a little typo
Initial attempt to validate context before running filters.

This way developers will see any potential missing context without
running the entire filter, which may not ever encounter a case where
it needs the context.
Merge pull request #21 from blackerby/small-typo-correction

Corrected a little typo
Cleaned up context validation. Declared requirements for EmojiFilter.

Declaring required contexts has been simplified.
validates_context_presence no longer requires self to be passed in.
Instead it will attempt to build the caller name by using
Kernel::caller.

Tests have been added for filters that require context.
Added required context info.

Noted in filters if a context is required. Updated README to include info about
required contexts.
Made constant look like code.