Add `exp2()` and `exp10()` functions

[nehill197]

Add transformation functions for 2^x and 10^x

Fixes #44

[nehill197]

@joethorley is this better? I need these additional lines to keep the same behaviour as exp():

• NAs pass through
• error if x is NULL or not numeric

[aylapear]

Update the docs

• remove details tag
• update details tag with value
• add a second example
• remove examples not of the function being written
• remove output of examples
• Full stops for the description sentence/paragraph, ie end sentences with a period
• Title case for Title tag, ie important words are capitalized and no period at end

Update tests

• split tests into their own test_that block with a descriptive name
• test various types (strings, integers, real)

[aylapear]

Looks good! Thanks for making those changes