Add comma as valid character in synopsis parser

Add a comma as a valid character in the value field of the synopsis parser.
Example:
[--allowed_themes=<themeA,themeB>]
fix test that was passing with the wrong error message
fix error message when fetching help for nested commands (fixes #1017)
set only the MYSQL_PWD environment variable

this avoids having to use the unreliable $_ENV superglobal

fixes #950
Merge pull request #1013 from sboisvert/master

Add comma as valid character in synopsis parser
split test scenario for help command (one for when wp is installed and other for when it is not)

see #1034
Merge pull request #1034 from rodrigoprimo/fix-help-error-msg

Fix error message when fetching help for nested commands
Add support for exporting specific tables
Dynamically build mysqldump command rather than repeating it
Add example for exporting specific tables
Merge pull request #1038 from CodeAwhile/db-export-tables

Add support for exporting specific tables
Whitelisted underscore in synopsis values.
Merge pull request #1039 from Rarst/synopsis-whitelist-underscore

Whitelisted underscore in synopsis values.