Select a piece of text with a cursor and call these functions as an add-in
to enclose the text with special symbols. Text between $.$ is
interpreted as an inline equation and between $$.$$ or \[.\]
as a block equation in R Markdown.
rmd_equation_inline() rmd_equation_block() rmd_equation_block2()
Other R Markdown formatting add-ins: format_rmd,
rmd_code_block, rmd_list