Add support for textDocument/onTypeFormatting

[rcjsuen]

We should add support for the textDocument/onTypeFormatting request.

If the user enters a \ or a ` and it is the escape character for the Dockerfile, the subsequent line should be formatted if necessary.

FROM busybox
EXPOSE 8080
8081

In the example above, if the user places a backslash at the end of the second line, the following line's 8081 should be indented.

#escape=`
FROM busybox
EXPOSE 8080
8081

In this second example above, placing a backslash on the third line will not cause a formatting request to be processed because the backslash is not the escape character. However, if the backtick is entered then the following line should be formatted.