msgplural index parsing logic is wrong

[rsun-wish]

https://github.com/chai2010/gettext-go/blob/master/po/message.go#L134

In the case where msgstr contains square brackets, the parsed index value would always be 0.

Example:

msgctxt "NAME OF BUTTON MERCHANTS CAN CLICK TO DOWNLOAD ALL THEIR PRODUCTS AS AN CSV "
"FILE. INCLUDED IS THE NAME OF THE WAREHOUSE AND THE NUMBER OF ROWS THAT WILL "
"BE IN THE FILE."
msgid "Download CSV [{%2=warehouse name}] ({%1=number of products} row)"
msgid_plural "Download CSV [{%2=warehouse name}] ({%1=number of products} rows)"
msgstr	[0] "Baix[1]ar CSV [{%2=warehouse name}] ({%1=number of products} linha)"
msgstr		[1] "Baix[2]ar CSV [{%2=warehouse name}] ({%1=number of products} linhas)"

The expected length of Message.MsgStrPlural should be 2 but the actual result is 1 because the message contains square brackets.