Makefile contains bashism's, causes failure to install on Ubuntu 18.04
When trying to install this on Ubuntu 18.04, it failed:
```
$ sudo make DESTDIR=/opt/taskopen install
gzip -c doc/man/taskopen.1 > doc/man/taskopen.1.gz
gzip -c doc/man/taskopenrc.5 > doc/man/taskopenrc.5.gz
sed s',#PATH_EXT=.*,&\nPATH_EXT=/usr/local//share/taskopen/scripts,' taskopen > taskopen.pl
mkdir -p /opt/taskopen//usr/local//bin
install -m 0755 taskopen.pl /opt/taskopen//usr/local//bin/taskopen
mkdir -p /opt/taskopen//usr/local//share/man/{man1,man5}
install -m 0644 doc/man/taskopen.1.gz /opt/taskopen//usr/local//share/man/man1/
install: target '/opt/taskopen//usr/local//share/man/man1/' is not a directory: No such file or directory
Makefile:13: recipe for target 'install' failed
make: *** [install] Error 1
```

This appears to be due to the Makefile containing some bash specific syntax, and Ubuntu 18.04's `/bin/sh` using `dash` instead of `bash`:
```
$ ls -l /bin/sh
lrwxrwxrwx 1 root root 4 May  4 11:43 /bin/sh -> dash*
```

I was able to workaround this issue by explicitly using `bash` when running `make`:
```
$ sudo make SHELL=/bin/bash PREFIX=/opt/taskopen install
gzip -c doc/man/taskopen.1 > doc/man/taskopen.1.gz
gzip -c doc/man/taskopenrc.5 > doc/man/taskopenrc.5.gz
sed s',#PATH_EXT=.*,&\nPATH_EXT=/opt/taskopen/share/taskopen/scripts,' taskopen > taskopen.pl
mkdir -p //opt/taskopen/bin
install -m 0755 taskopen.pl //opt/taskopen/bin/taskopen
mkdir -p //opt/taskopen/share/man/{man1,man5}
install -m 0644 doc/man/taskopen.1.gz //opt/taskopen/share/man/man1/
install -m 0644 doc/man/taskopenrc.5.gz //opt/taskopen/share/man/man5/
mkdir -p //opt/taskopen/share/taskopen/doc/html
install -m 0644 doc/html/* //opt/taskopen/share/taskopen/doc/html
mkdir -p //opt/taskopen/share/taskopen/scripts
install -m 755 scripts/* //opt/taskopen/share/taskopen/scripts/
```

Since the default shell for `make` is `/bin/sh`, I think the bash-isms should either be removed from the Makefile, or the README should be updated such that it states that `SHELL=/bin/bash` should be specified.

With all that said, thanks for the awesome tool! :)