Add support for passing minimum_perc value for .tx/config

[rffontenelle]

sphinx-intl update-txconfig will add resources to .tx/config with minimum_prec using default 0, which causes tx to pull all PO files even if 0% translated. Under the hood this command uses tx add command which surprisingly enough doesn't provide an option for setting minimum_prec (tx pull has though).

Example of .tx/config entry of .tx/config as currently generated by update-txconfig:

[main]
host = https://www.transifex.com

[o:python-doc:p:future-python-newest:r:about]
file_filter            = ./<lang>/LC_MESSAGES/about.po
source_file            = pot/about.pot
type                   = PO
minimum_perc           = 0
resource_name          = about
replace_edited_strings = false
keep_translations      = false

Currently, an alternative solution is sed -i '/^minimum_perc/s/0/1/' .tx/config.

My suggestion is to add to sphinx-intl update-txconfig a command-line option --minimum-perc that takes an int (defaulting to 0) and then replaces 0 with the value passed right after running tx add command.