#359 SshConsole: expand variables like System console

[Ben Golding]

Would like variables which are supported in the System console to also work in SshConsole.
Specifically ${d} ${f} ${p} and ${n} would be useful.

For ${d} ${f} if the current buffer is an SFTP remote file, the path should be stripped of the prefix e.g.
sftp://user@host.domain.com:22/path/to/Class.java
becomes
/path/to/Class.java

This might also be useful for ${p} I'm not sure as I don't use ProjectViewer.

For the sake of paranoia/compatibility, I would also be fine to use a different variable name for the "stripped" version of these variables.

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jEdit 5.3.0 (stable)
Enabled plugins: FTP 1.3, Common Controls 1.7.4, Console 5.1.4, ErrorList 2.3, SshConsole 1.0.7
Windows 7 x86_64
Oracle JRE 1.8.0_66 (x86_64) + JCE unlimited strength policies