export command should support multiple "extras" in single flag, similar to install command

[johnmacnamararseg]

• I am on the latest Poetry version.
• I have searched the issues of this repo and believe that this is not a duplicate.

• OS version and name: macOS Catalina (10.15.3) (Issue is not OS dependent)
• Poetry version: 1.1.3

Issue

This works for defining extras with install command

poetry install --extras "mysql pgsql"
poetry install -E mysql -E pgsql

however, -E/--extras option of export command does not support defining multiple extras with a single flag. For instance,

pyproject.toml

.
.
.
mysql = { version = "^0.0.1", optional = true }
pgsql = { version = "^1.2", optional = true }

[tool.poetry.extras]
mysql  = ["mysql"]
pgsql = ["pgsql"]

poetry export -E mysql -E pgsql
or 
poetry export --extras mysql --extras pgsql

will work, but

poetry export --extras "mysql pgsql"

is not supported and neither extra will be included in the export.

Issue is pretty minor, but difference in behavior causes some confusion.

I am opening a PR to add the ability to define multiple extras in a single flag in the export command