Allow choice of character to replace space

[Jmuccigr]

Is your feature request related to a problem? Please describe.

Not a problem as much as a preference. Right now spaces in various places (e.g., tags) get converted to dashes when "remove_spaces" is true.

Describe the solution you'd like

I'd like the option of choosing which character the space gets replaced by. I prefer an underscore, but I can't set that - which is perhaps ironic, given its use in mdnotes' own preferences. :-)

[astroHaoPeng]

Something like

extensions.mdnotes.tags.remove_spaces = "true"
extensions.mdnotes.tags.replace_space_by = "-"

will be great. Thanks.

Or maybe easier, just allowing to pass "replace_space_by"="_" to the custom formatter.