Adding styles specifically for blog images

- Fixes #604

New blog image use cases:

Default use case (max-width: 100%)
`{% img path/to/image/in/_assets/file.png %}`

Float right:
`{% img right path/to/image/in/_assets/file.png %}`

Float left:
`{% img left path/to/image/in/_assets/file.png %}`

center:
`{% img center path/to/image/in/_assets/file.png %}`

Full (force 100% width):
`{% img full path/to/image/in/_assets/file.png %}`

Matte (useful for white bg images)
`{% img matte path/to/image/in/_assets/file.png %}`

Combine for fun:
`{% img matte right path/to/image/in/_assets/file.png %}`
`{% img matte full path/to/image/in/_assets/file.png %}`