Only load the condarc file once
Add -c/--channel and --override-channels

These were added to all the commands that use channels, namely install,
remove, create, update, and search.  search already has -c for --canonical, so
the -c shortcut was not created there.

The syntax is

conda install -c channel1 -c channel2 package1 package2

This will prepend channel1 and channel2 to the channel search list from
.condarc, or the defaults if there is no .condarc.  To instead replace these,
add the --override-channels flag.  This can also be used in conjunction with
the defaults or rc meta-channels to change the prepend behavior.  For example,
to search first the channels in .condarc, then channel1, use

conda install -c rc -c channel1 package