Add '--init-command', '-C' to the command line switches.

In order to allow the execution of commands before dropping to an
interactive prompt, a new switch, '-C' or '--init-command' has been
added to those switches that we accept.

The documentation has been updated correspondingly.

The original code only supported a single command list to be executed,
and this command list terminates the shell when it completes. To allow
the new command list to preceed the original one, both have been
wrapped in a new container class 'command_line_switches_t'. This is
then passed around in place of the list of strings we used previously.

I had considered moving the interactive, login and other command line
switch states into this container, but doing so would change far more
of the code, moving the structure to be available globally, and I
wasn't confident of the impact. However, this might be a useful thing
to do in the future.

A new function, run_command_list, was lifted from the prior execution
code, and re-used for both the initial command and the regular command
execution.

doc_src/fish.txt

@@ -13,6 +13,8 @@ The following options are available:
 
 - `-c` or `--command=COMMANDS` evaluate the specified commands instead of reading from the commandline
 
+- `-C` or `--init-command=COMMANDS` evaluate the specified commands after reading the configuration, before running the command specified by `-c` or reading interactive input
+
 - `-d` or `--debug-level=DEBUG_LEVEL` specify the verbosity level of fish. A higher number means higher verbosity. The default level is 1.
 
 - `-i` or `--interactive` specify that fish is to run in interactive mode