highlighting commands does not consider environment variables

[euclio]

If you start a command with an environment variable, the command is highlighted as if it is invalid.

$ /bin/sh     # highlighted correctly
$ export BIN=/bin
$ $BIN/sh   # highlighted with the "invalid command" highlighting, even though `$BIN` contains a valid path

[phy1729]

The export is unrelated. The issue is that the block handling parameters (et seq) only handles $foo or ${foo}. Looks like a dup of #484.

[danielshahaf]

Duplicate of #484, with virtually the same example, even.