IMPALA-601: Read shell configuration from a file

Optionally loads options from a file in the user's home directory, called
'.impalarc', though the path to another file can be passed in as a command-line
option. The file must have a case-sensitive [impala] header. Specifying
the option in the command line overwrites the config file's value
for the option for that instance of the shell. If an option is not
specified in the config file, its default value is used.

Change-Id: I218da2c1e10308c5b8729883fa625f0c284397a7
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2956
Reviewed-by: Abdullah Yousufi <abdullah.yousufi@cloudera.com>
Tested-by: jenkins