Introduce a switch to break from recite() if the forked process returns
a non-zero exit code. The one except to this, is when using semicolons,
then we do not care if the previous command failed.

Reaping is currently handled by a signal handler, so this code is no
longer necessary.

Removes the custom errors in src/recite/erro.rs, and replaces them with
std::io::Errors throughout (recite(), incant_, macros).

Fixed a bug with the way forking to the background is handled, where
registering the signal handler in main for all processes would break
couplets (i.e. pipes). Instead, this sets up a new signal handler each
time a process is forked into the background. It uses a Vec<i32> to keep
track of all the background processes.

First off, moved the giant match statements out of recite(), and into
macros in src/recite/ps.rs. There still needs to be two, since any verse
using the 'couplet' meter will need to redirect its STDOUT. Now the
recite() function returns a Result<(), Mishap>, which can be invoked
when calling the incant_ functions.

Custom errors were added in the form of 'Mishap''s. They are intended to
be returned from the incant_ functions, in the event that something goes
wrong with the Command::spawn() or Child::wait(). They each take a
String, which should be the verb or stanza that was entered by the user.

The incant_ functions separate the functionality of each type of meter
from the recite() function. They return a Result<i32, Mishap>, where
i32 is the exit code of the program that ran, and Mishap is a possible
error.

Before, the shell was cheating at forking a process to the background.
It would actually spawn a thread to wait for that process to finish.
Now, the program simply registers a handler for SIGCHLD, and uses libc's
waitpid() function to reap the child process, and print some output to
the user, indicating that it's finished.

Replaced the 'ctrlc' crate with 'signal-hook' for handling of SIGINT.
The 'signal_hook::low_level::register' function is actually unsafe.
However, according to
https://docs.rs/signal-hook/latest/signal_hook/low_level/fn.register.html,
it is only unsafe in the case of multithreaded applications. There are
some race conditions as well. For instance, it appears that even when we
fork to a child process, SIGINT is captured on both that process, as
well as the shell.

Broke out the structs for a poem into their own file: src/recite.rs.
Also put the 'prefresh' function into it's own file: src/recite/path.rs.
Commented most of the parser code (including structs and helper methods
related to parsing (i.e. Verse, Stanza, Meter, Poem)). Renamed any
instance of the 'paths' variable to 'path'.

Add back change directory functionality into the new parser.

Now the parser goes char by char, since special characters like '|' and
'&' don't necessarily have to be whitespace seperated. Also added some
VERY basic error detection for the parser (revolving around special
chars).

This adds some preliminary support for pipes (|), forks (&), and
consecutive command calls (&&) to the shell.

Only print out the command name in error messages, rather than printing
out the full path to the command.