A very simple backup plugin for holland that allows executing arbitrary scripts

This replaces a single variable called ${backupdir} with the actual
holland backup directory.

A command is considered as failed if it exits with non-zero status.

Note: This plugin currently returns a backup size estimate of 1 byte.

Example configuration:

[holland:backup]
plugin = script

[script]
shell = /bin/sh
estimation-method = const:64M
cmd = innobackupex-1.5.1 --stream=tar4ibd --slave-info /tmp > ${backupdir}/xtrabackup.tar.gz

More complex:

[holland:backup]
plugin = script

[script]
shell = /bin/bash
estimation-method = const:64M
# exit with mysqldump exit status rather than gzip's
cmd = mysqldump --all-databases | gzip --fast > ${backupdir}/all_databases.sql.gz; exit ${PIPESTATUS[0]}

Estimation method can be set to one of:

dir:<path>  - e.g. dir:/var/lib/mysql

cmd:<cmdline> - e.g. echo 3.14159G

const:<constant> - e.g. 4G

Both cmd and const work with strings of the format:

N[.N][BKMGTPE]

If no units are specified the units are assumed to be bytes

4 	= 4 bytes
4B 	= 4 bytes
4K	= 4096 bytes
4M	= 4194304 bytes
4G 	= 4294967296 bytes
4.5G	= 4831838208.0 bytes

cmd should output a string matching this output and const should be a simple 
string matching this output.  Should that output not match this format the
estimation will fail and cause the backup to fail.