Added extra_opts support

Added simple support for specifying additional options to the ctags process. This is handy when running against a mixed codebase, or when you want to exclude large directories containing mostly binary code.

To specify options, just add an extra_opts parameter in the CTags plugin user settings, containing an array of strings to append to the ctags command. For example, mine contains the following options:

{
    "extra_opts" : ["--languages=C++", "--exclude=.git", "--exclude=build", "--exclude=out"]
}