Add changed environment variable

[mvochoa]

An environment variable was added with the file path that was modified, so that the user can use it in the commands.

$ CompileDaemon --build="go build \$FILE"

In addition, the help message was modified:

$ CompileDaemon --help
Copyright (c) 2013, Marian Tietz
Examples
In its simplest form, the defaults will do. With the current working directory set
to the source directory you can simply…
$ CompileDaemon
… and it will recompile your code whenever you save a source file.
If you want it to also run your program each time it builds you might add…
$ CompileDaemon -command="./MyProgram -my-options"
… and it will also keep a copy of your program running. Killing the old one and
starting a new one each time you build. For advanced usage you can also supply
the changed file to the command by doing…
$ CompileDaemon -command="./MyProgram -my-options %[1]s"
…but note that this will not be set on the first start.
You may find that you need to exclude some directories and files from
monitoring, such as a .git repository or emacs temporary files…
$ CompileDaemon -exclude-dir=.git -exclude=".#*"
If you want to monitor files other than .go and .c files you might…
$ CompileDaemon -include=Makefile -include="*.less" -include="*.tmpl"
If you only need to work with the file that changes, the $FILE variable has the modified file path
$ CompileDaemon --build="go build \$FILE"
Usage of CompileDaemon:
-build string
Command to rebuild after changes (default "go build")
-build-dir string
Directory to run build command in. Defaults to directory
-color
Colorize output for CompileDaemon status messages
-command string
Command to run and restart after build
-command-stop
Stop command before building
-directory string
Directory to watch for changes (default ".")
-exclude value
Don't watch files matching this name -exclude-dir value Don't watch directories matching this name
-graceful-kill
Gracefully attempt to kill the child process by sending a SIGTERM first
-graceful-timeout uint
Duration (in seconds) to waitfor graceful kill to complete (default 3)
-include value
Watch files matching this name
-log-prefix
Print log timestamps and subprocess stderr/stdout output (default true)
-pattern string
Pattern of watched files (default "(.+\\.go|.+\\.c)$")
-recursive
Watch all dirs. recursively (default true)
-run-dir string
Directory to run command in. Defaults to directory
-verbose
Be verbose about which directories are watched.

[githubnemo]

Hi, thanks for your PR and sorry for the wait.

This seems to be a sensible addition but it is quite hard to review the relevant portions of the change among all the other changes. I will take a look, though.