Provide workaround for jinja macros that use reserved words

[tconbeer]

Describe the bug
jinjafmt passes jinja to black, but jinja allows some things that python does not. Specifically, variables (esp. macro arguments) can have names that are reserved words. For example:

{{ dbt_utils.star(from=ref('snowplow_page_views_30'), except=fields_to_exclude|upper) }}

This is valid jinja and dbt sql (from gitlab's project), but from and except are both reserved words in python. Running black -c "from" returns error: cannot format <string>: Cannot parse: 1:4: from

Right now jinjafmt no-ops in these situations, but it wouldn't be that hard to replace these reserved words (followed by an eq, e.g. from=) with a sentinel (e.g., from__sqlfmt__=), format that, and then replace the sentinel with the keyword again. There are 33 reserved words in python

[tconbeer]

This could also provide a solution to #155 -- return is a reserved word, but jinja allows it as a function name. So we'd probably want to replace reserved words followed by parens, in addition to equals

[tconbeer]

Also consider replacing ~ (jinja string concatenation) with + tilde + or similar?