zsh: prevent warning message on old zsh.

Trying to determine whether a value is a number by using it in arithmetic
expressions makes zsh complain if it is not, in fact, a number.  Use grep to
check this instead, since it is more robust and less likely to complain.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
zsh: don't always print colors as bold.

Due to a bug in the color_fg function, colors in 16-color terminals using
zsh before 4.3.9 were always bold.  Fix this.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>